Thread: [Matplotlib-users] using color as z-axis for plots

I've got many series of data that I want to plot, and each has an  
additional scalar that is valid for the whole series.  What I want to  
do is plot all these series on top of each other (plot can do this  
just fine), but with the additional scalar changing the color,  
efectively using color as the z-axis.  I'm not seeing how to do that.   
If there was a function where I could give a color map a value and it  
would spit out the color, that would work, but I haven't seen it.   
Thanks for your help.


--Chad Kidder

On Mon, Dec 8, 2008 at 12:45 PM, Chad Kidder <cck...@gm...> wrote:
> I've got many series of data that I want to plot, and each has an
> additional scalar that is valid for the whole series.  What I want to
> do is plot all these series on top of each other (plot can do this
> just fine), but with the additional scalar changing the color,
> efectively using color as the z-axis.  I'm not seeing how to do that.
> If there was a function where I could give a color map a value and it
> would spit out the color, that would work, but I haven't seen it.
> Thanks for your help.


Check out the scatter_demo -- scatter takes an optional argument 'c'
for the color and an optional colormap

http://matplotlib.sourceforge.net/users/screenshots.html#scatter-demo




>
> --Chad Kidder
>
>
>
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Chad Kidder wrote:
> I've got many series of data that I want to plot, and each has an  
> additional scalar that is valid for the whole series.  What I want to  
> do is plot all these series on top of each other (plot can do this  
> just fine), but with the additional scalar changing the color,  
> efectively using color as the z-axis.  I'm not seeing how to do that.   
> If there was a function where I could give a color map a value and it  
> would spit out the color, that would work, but I haven't seen it.   
> Thanks for your help.

Try using scatter instead of plot.  Specifically, the 'c' keyword argument:

       *c*:
         a color. *c* can be a single color format string, or a
         sequence of color specifications of length *N*, or a
         sequence of *N* numbers to be mapped to colors using the
         *cmap* and *norm* specified via kwargs (see below). Note
         that *c* should not be a single numeric RGB or RGBA
         sequence because that is indistinguishable from an array
         of values to be colormapped.  *c* can be a 2-D array in
         which the rows are RGB or RGBA, however.

For example:

import numpy as np
import matplotlib.pyplot as plt

x = np.random.randn(100)
y = np.random.randn(100)
data = x**2 + y**2
plt.scatter(x, y, c=data)
plt.show()

Ryan

-- 
Ryan May
Graduate Research Assistant
School of Meteorology
University of Oklahoma

On Dec 8, 2008, at 12:55 PM, Ryan May wrote:

> Chad Kidder wrote:
>> I've got many series of data that I want to plot, and each has an   
>> additional scalar that is valid for the whole series.  What I want  
>> to  do is plot all these series on top of each other (plot can do  
>> this  just fine), but with the additional scalar changing the  
>> color,  efectively using color as the z-axis.  I'm not seeing how  
>> to do that.   If there was a function where I could give a color  
>> map a value and it  would spit out the color, that would work, but  
>> I haven't seen it.   Thanks for your help.
>
> Try using scatter instead of plot.  Specifically, the 'c' keyword  
> argument:
>
>      *c*:
>        a color. *c* can be a single color format string, or a
>        sequence of color specifications of length *N*, or a
>        sequence of *N* numbers to be mapped to colors using the
>        *cmap* and *norm* specified via kwargs (see below). Note
>        that *c* should not be a single numeric RGB or RGBA
>        sequence because that is indistinguishable from an array
>        of values to be colormapped.  *c* can be a 2-D array in
>        which the rows are RGB or RGBA, however.
>
> For example:
>
> import numpy as np
> import matplotlib.pyplot as plt
>
> x = np.random.randn(100)
> y = np.random.randn(100)
> data = x**2 + y**2
> plt.scatter(x, y, c=data)
> plt.show()
>
> Ryan
>

Close, but not quite what I want.  Maybe this will tell what I want to  
do better:

----------------------
import numpy as n
import matplotlib.pyplot as p

nlines = 100
z = n.random.rand(nlines)
x = n.array(range(nlines))
t1, t2 = n.meshgrid(x,z)
y = t1+t2-0.5
for ii in range(nlines):
    p.plot(x,y[ii,:],color = str(z[ii]))
p.show()
-------------------
Instead of getting a grayscale plot out, I'd like to use a colormap  
like jet() or winter().  Any ideas there?





--Chad Kidder

On Mon, Dec 8, 2008 at 1:25 PM, Chad Kidder <cck...@gm...> wrote:
-------------------
> Instead of getting a grayscale plot out, I'd like to use a colormap
> like jet() or winter().  Any ideas there?

How about?

  import matplotlib.cm as cm

  for ii in range(nlines):
      color = cm.jet(z[ii])
      p.plot(x,y[ii,:],color=color)

All of the mpl colormaps are callable, so if you pass in a [0..1]
normalized value they will return an RGB tuple

JDH

Chad Kidder wrote:
> On Dec 8, 2008, at 12:55 PM, Ryan May wrote:
> 
>> Chad Kidder wrote:
>>> I've got many series of data that I want to plot, and each has an   
>>> additional scalar that is valid for the whole series.  What I want  
>>> to  do is plot all these series on top of each other (plot can do  
>>> this  just fine), but with the additional scalar changing the  
>>> color,  efectively using color as the z-axis.  I'm not seeing how  
>>> to do that.   If there was a function where I could give a color  
>>> map a value and it  would spit out the color, that would work, but  
>>> I haven't seen it.   Thanks for your help.
>> Try using scatter instead of plot.  Specifically, the 'c' keyword  
>> argument:
>>
>>      *c*:
>>        a color. *c* can be a single color format string, or a
>>        sequence of color specifications of length *N*, or a
>>        sequence of *N* numbers to be mapped to colors using the
>>        *cmap* and *norm* specified via kwargs (see below). Note
>>        that *c* should not be a single numeric RGB or RGBA
>>        sequence because that is indistinguishable from an array
>>        of values to be colormapped.  *c* can be a 2-D array in
>>        which the rows are RGB or RGBA, however.

Here's what you're looking for:

import numpy as n
import matplotlib.pyplot as p
import matplotlib.colors as mcolors
import matplotlib.cm as cm

cmap = cm.get_cmap('winter')
norm = mcolors.Normalize(0, 1) #Range of z

nlines = 100
z = n.random.rand(nlines)
x = n.arange(nlines)
t1, t2 = n.meshgrid(x,z)
y = t1+t2-0.5

#Uses normalize to map z values to range of 0 to 1.
#Cmap maps these normalized values to colors
colors = cmap(norm(z))
for ii in range(nlines):
     p.plot(x, y[ii], color=colors[ii])
p.show()

Ryan

-- 
Ryan May
Graduate Research Assistant
School of Meteorology
University of Oklahoma

On Dec 8, 2008, at 2:00 PM, Ryan May wrote:

> Chad Kidder wrote:
>> On Dec 8, 2008, at 12:55 PM, Ryan May wrote:
>>> Chad Kidder wrote:
>>>> I've got many series of data that I want to plot, and each has  
>>>> an   additional scalar that is valid for the whole series.  What  
>>>> I want  to  do is plot all these series on top of each other  
>>>> (plot can do  this  just fine), but with the additional scalar  
>>>> changing the  color,  efectively using color as the z-axis.  I'm  
>>>> not seeing how  to do that.   If there was a function where I  
>>>> could give a color  map a value and it  would spit out the color,  
>>>> that would work, but  I haven't seen it.   Thanks for your help.
>>> Try using scatter instead of plot.  Specifically, the 'c' keyword   
>>> argument:
>>>
>>>     *c*:
>>>       a color. *c* can be a single color format string, or a
>>>       sequence of color specifications of length *N*, or a
>>>       sequence of *N* numbers to be mapped to colors using the
>>>       *cmap* and *norm* specified via kwargs (see below). Note
>>>       that *c* should not be a single numeric RGB or RGBA
>>>       sequence because that is indistinguishable from an array
>>>       of values to be colormapped.  *c* can be a 2-D array in
>>>       which the rows are RGB or RGBA, however.
>
> Here's what you're looking for:
>
> import numpy as n
> import matplotlib.pyplot as p
> import matplotlib.colors as mcolors
> import matplotlib.cm as cm
>
> cmap = cm.get_cmap('winter')
> norm = mcolors.Normalize(0, 1) #Range of z
>
> nlines = 100
> z = n.random.rand(nlines)
> x = n.arange(nlines)
> t1, t2 = n.meshgrid(x,z)
> y = t1+t2-0.5
>
> #Uses normalize to map z values to range of 0 to 1.
> #Cmap maps these normalized values to colors
> colors = cmap(norm(z))
> for ii in range(nlines):
>    p.plot(x, y[ii], color=colors[ii])
> p.show()
>
> Ryan

Thanks all.  I had tried something close, but it didn't work.  This  
works great.

Chad Kidder wrote:
> On Dec 8, 2008, at 12:55 PM, Ryan May wrote:
> 
>> Chad Kidder wrote:
>>> I've got many series of data that I want to plot, and each has an   
>>> additional scalar that is valid for the whole series.  What I want  
>>> to  do is plot all these series on top of each other (plot can do  
>>> this  just fine), but with the additional scalar changing the  
>>> color,  efectively using color as the z-axis.  I'm not seeing how  
>>> to do that.   If there was a function where I could give a color  
>>> map a value and it  would spit out the color, that would work, but  
>>> I haven't seen it.   Thanks for your help.
>> Try using scatter instead of plot.  Specifically, the 'c' keyword  
>> argument:
>>
>>      *c*:
>>        a color. *c* can be a single color format string, or a
>>        sequence of color specifications of length *N*, or a
>>        sequence of *N* numbers to be mapped to colors using the
>>        *cmap* and *norm* specified via kwargs (see below). Note
>>        that *c* should not be a single numeric RGB or RGBA
>>        sequence because that is indistinguishable from an array
>>        of values to be colormapped.  *c* can be a 2-D array in
>>        which the rows are RGB or RGBA, however.
>>
>> For example:
>>
>> import numpy as np
>> import matplotlib.pyplot as plt
>>
>> x = np.random.randn(100)
>> y = np.random.randn(100)
>> data = x**2 + y**2
>> plt.scatter(x, y, c=data)
>> plt.show()
>>
>> Ryan
>>
> 
> Close, but not quite what I want.  Maybe this will tell what I want to  
> do better:
> 
> ----------------------
> import numpy as n
> import matplotlib.pyplot as p
> 
> nlines = 100
> z = n.random.rand(nlines)
> x = n.array(range(nlines))
> t1, t2 = n.meshgrid(x,z)
> y = t1+t2-0.5
> for ii in range(nlines):
>     p.plot(x,y[ii,:],color = str(z[ii]))
> p.show()
> -------------------
> Instead of getting a grayscale plot out, I'd like to use a colormap  
> like jet() or winter().  Any ideas there?

In [6]:cmap = get_cmap('jet')

In [7]:cmap(0.2)
Out[7]:(0.0, 0.29999999999999999, 1.0, 1.0)

In [8]:cmap(0.8)
Out[8]:(1.0, 0.40740740740740755, 0.0, 1.0)


The pyplot.get_cmap() function gets a colormap by name.  Calling that 
colormap with a floating-point argument in the 0-1 range returns the 
mapped color as an rgba tuple, which will be accepted by the color kwarg 
of plot.  You can use pyplot.normalize to map your z range to the 0-1 range:

In [2]:norm = normalize(vmin=2, vmax=4)

In [3]:norm(3)
Out[3]:0.5


Alternatively, you can use a LineCollection.  See the 
examples/pylab_examples/line_collection2.py script.

Eric




> 
> 
> 
> 
> 
> --Chad Kidder
> 
> ------------------------------------------------------------------------------
> SF.Net email is Sponsored by MIX09, March 18-20, 2009 in Las Vegas, Nevada.
> The future of the web can't happen without you.  Join us at MIX09 to help
> pave the way to the Next Web now. Learn more and register at
> http://ad.doubleclick.net/clk;208669438;13503038;i?http://2009.visitmix.com/
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