Thread: [Matplotlib-users] plot color as a function of values?

Hi,

Imagine you have something like:

from pylab import *
t = arange(0.0, 2.0, 0.01)
s = sin(2*pi*t)
ax = subplot(111)
ax.plot(t, s)

That's fine but now I would like to plot the negative parts of the curve 
in red and the positive one in green.
Is there a nice pylab oriented way to do that? Some kind of "conditional 
formating"?

Xavier

Xavier Gnata wrote:
> Hi,
> 
> Imagine you have something like:
> 
> from pylab import *
> t = arange(0.0, 2.0, 0.01)
> s = sin(2*pi*t)
> ax = subplot(111)
> ax.plot(t, s)
> 
> That's fine but now I would like to plot the negative parts of the curve 
> in red and the positive one in green.
> Is there a nice pylab oriented way to do that? Some kind of "conditional 
> formating"?

Not built in, but you can do it easily with masked arrays.  See 
http://matplotlib.sourceforge.net/examples/pylab_examples/masked_demo.html

It is not exactly what you want, but close:

sneg = np.ma.masked_greater_equal(s, 0)
spos = np.ma.masked_less_equal(s, 0)
ax.plot(t, spos, 'g')
ax.plot(t, sneg, 'r')

What this does not do is ensure that there is no gap where the line 
crosses zero.  For that, you would need to ensure that your sampling of 
s(t) includes the zeros.

Eric


> 
> Xavier
> 
> ------------------------------------------------------------------------------
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Eric Firing wrote:
> Xavier Gnata wrote:
>> Hi,
>>
>> Imagine you have something like:
>>
>> from pylab import *
>> t = arange(0.0, 2.0, 0.01)
>> s = sin(2*pi*t)
>> ax = subplot(111)
>> ax.plot(t, s)
>>
>> That's fine but now I would like to plot the negative parts of the 
>> curve in red and the positive one in green.
>> Is there a nice pylab oriented way to do that? Some kind of 
>> "conditional formating"?
>
> Not built in, but you can do it easily with masked arrays.  See 
> http://matplotlib.sourceforge.net/examples/pylab_examples/masked_demo.html 
>
>
> It is not exactly what you want, but close:
>
> sneg = np.ma.masked_greater_equal(s, 0)
> spos = np.ma.masked_less_equal(s, 0)
> ax.plot(t, spos, 'g')
> ax.plot(t, sneg, 'r')
>
> What this does not do is ensure that there is no gap where the line 
> crosses zero.  For that, you would need to ensure that your sampling 
> of s(t) includes the zeros.
>
> Eric

It should do the trick because my sampling is very high.

Xavier

Maybe a little shorter is the where() keyword, and even that can be omitted:

ax.plot(t[where(s>=0)],s[where(s>=0)],"g")
ax.plot(t[where(s<0)],s[where(s<0)],"r")

or, shorter:

ax.plot(t[s>=0],s[s>=0],"g")
ax.plot(t[s<0],s[s<0],"r")

cheers

Thomas



Xavier Gnata-2 wrote:
> 
> Hi,
> 
> Imagine you have something like:
> 
> from pylab import *
> t = arange(0.0, 2.0, 0.01)
> s = sin(2*pi*t)
> ax = subplot(111)
> ax.plot(t, s)
> 
> That's fine but now I would like to plot the negative parts of the curve 
> in red and the positive one in green.
> Is there a nice pylab oriented way to do that? Some kind of "conditional 
> formating"?
> 
> Xavier
> 
> ------------------------------------------------------------------------------
> Come build with us! The BlackBerry(R) Developer Conference in SF, CA
> is the only developer event you need to attend this year. Jumpstart your
> developing skills, take BlackBerry mobile applications to market and stay 
> ahead of the curve. Join us from November 9 - 12, 2009. Register now!
> http://p.sf.net/sfu/devconference
> _______________________________________________
> Matplotlib-users mailing list
> Mat...@li...
> https://lists.sourceforge.net/lists/listinfo/matplotlib-users
> 
> 

-- 
View this message in context: http://www.nabble.com/plot-color-as-a-function-of-values--tp25848622p25858967.html
Sent from the matplotlib - users mailing list archive at Nabble.com.

ax.plot(t[s>=0],s[s>=0],"g")
ax.plot(t[s<0],s[s<0],"r")


Whaou! That's what I call a nice pythonic syntax.

XAvier

> Maybe a little shorter is the where() keyword, and even that can be omitted:
>
> ax.plot(t[where(s>=0)],s[where(s>=0)],"g")
> ax.plot(t[where(s<0)],s[where(s<0)],"r")
>
> or, shorter:
>
> ax.plot(t[s>=0],s[s>=0],"g")
> ax.plot(t[s<0],s[s<0],"r")
>
> cheers
>
> Thomas
>
>
>
> Xavier Gnata-2 wrote:
>   
>> Hi,
>>
>> Imagine you have something like:
>>
>> from pylab import *
>> t = arange(0.0, 2.0, 0.01)
>> s = sin(2*pi*t)
>> ax = subplot(111)
>> ax.plot(t, s)
>>
>> That's fine but now I would like to plot the negative parts of the curve 
>> in red and the positive one in green.
>> Is there a nice pylab oriented way to do that? Some kind of "conditional 
>> formating"?
>>
>> Xavier
>>
>> ------------------------------------------------------------------------------
>> Come build with us! The BlackBerry(R) Developer Conference in SF, CA
>> is the only developer event you need to attend this year. Jumpstart your
>> developing skills, take BlackBerry mobile applications to market and stay 
>> ahead of the curve. Join us from November 9 - 12, 2009. Register now!
>> http://p.sf.net/sfu/devconference
>> _______________________________________________
>> Matplotlib-users mailing list
>> Mat...@li...
>> https://lists.sourceforge.net/lists/listinfo/matplotlib-users
>>
>>
>>     
>
>   

Xavier Gnata wrote:
> ax.plot(t[s>=0],s[s>=0],"g")
> ax.plot(t[s<0],s[s<0],"r")
> 

I don't think it does what you want, though, unless you are plotting 
markers, not lines.  With lines, you will have line segments 
approximately on the x-axis across the gaps.  Using masked arrays avoids 
that.  The gaps (masked segments) will not be stroked.

Eric

> 
> Whaou! That's what I call a nice pythonic syntax.
> 
> XAvier
> 
>> Maybe a little shorter is the where() keyword, and even that can be omitted:
>>
>> ax.plot(t[where(s>=0)],s[where(s>=0)],"g")
>> ax.plot(t[where(s<0)],s[where(s<0)],"r")
>>
>> or, shorter:
>>
>> ax.plot(t[s>=0],s[s>=0],"g")
>> ax.plot(t[s<0],s[s<0],"r")
>>
>> cheers
>>
>> Thomas
>>
>>
>>
>> Xavier Gnata-2 wrote:
>>   
>>> Hi,
>>>
>>> Imagine you have something like:
>>>
>>> from pylab import *
>>> t = arange(0.0, 2.0, 0.01)
>>> s = sin(2*pi*t)
>>> ax = subplot(111)
>>> ax.plot(t, s)
>>>
>>> That's fine but now I would like to plot the negative parts of the curve 
>>> in red and the positive one in green.
>>> Is there a nice pylab oriented way to do that? Some kind of "conditional 
>>> formating"?
>>>
>>> Xavier
>>>
>>> ------------------------------------------------------------------------------
>>> Come build with us! The BlackBerry(R) Developer Conference in SF, CA
>>> is the only developer event you need to attend this year. Jumpstart your
>>> developing skills, take BlackBerry mobile applications to market and stay 
>>> ahead of the curve. Join us from November 9 - 12, 2009. Register now!
>>> http://p.sf.net/sfu/devconference
>>> _______________________________________________
>>> Matplotlib-users mailing list
>>> Mat...@li...
>>> https://lists.sourceforge.net/lists/listinfo/matplotlib-users
>>>
>>>
>>>     
>>   
> 
> 
> ------------------------------------------------------------------------------
> Come build with us! The BlackBerry(R) Developer Conference in SF, CA
> is the only developer event you need to attend this year. Jumpstart your
> developing skills, take BlackBerry mobile applications to market and stay 
> ahead of the curve. Join us from November 9 - 12, 2009. Register now!
> http://p.sf.net/sfu/devconference
> _______________________________________________
> Matplotlib-users mailing list
> Mat...@li...
> https://lists.sourceforge.net/lists/listinfo/matplotlib-users