Thread: [Matplotlib-users] Disable Colorbar Scaling In Contour Plot

Problem 1: I have a dataset which has more values on one side of the scale
than the other in a contour map. Matplotlib will see that I have more values
towards one side of the scale, and put all of the colors in that color map
to that side of the scale.

For example, lets say I would like my colorbar to show values from -10^6 all
the way to 10^6.
But, most of my values are in the -10^6 to -10^5 range. How do I get my
colorbar not to automatically send all of the colors to that portion of the
colorbar and leave everything greater than -10^5 to have one color.

I would like to have the colorbar displaying the same color scaling
regardless of the data input.

Here is a copy of my code:
v2 =
[-1000000,-100000,-10000,-1000,-100,-10,0,10,100,1000,10000,100000,1000000]
	plt.subplot(6,1,2)
	cset4 = plt.contourf(x2,z2,k2,v2,cmap=cm.get_cmap('jet', len(v2)-1))
	cset5 = plt.contour(x2,z2,k2,v2,colors = 'k',linewidths = .22,hold='on')
	#set axes values
	plt.axis([xmin2,xmax2, zmin,zmax])
	#set y axes label
	plt.ylabel('Z, [km]')
	#display title
	plt.title('v3 Plot              Max:'+str("%.2f" %k2.max())+'		   Min:
'+str("%.2f" %k2.min()),size='small')
	#colorbar manipulation
	cbar = plt.colorbar(cset4,spacing="uniform", shrink =
0.95,ticks=[-1000000,-100000,-10000,-1000,-100,-10,0,10,100,1000,10000,100000,1000000])

cbar.ax.set_yticklabels(['-10^5','-10^4','-10^3','-10^2','-10^1',0,'10^1','10^2','10^3','10^4','10^5'])
	#close file
	f2.close()

Problem 2: If I have a colorbar scale which goes from -10 to 5, and I use
one of the standard colormaps which has white in the center of it. How do I
keep the zero point at the white color instead of the colorbar automatically
picking the midpoint for zero?

Problem 1 is much more of a pressing issue right now than problem 2.

Thank you 

-- 
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2010/8/11 tgabriel <tra...@gm...>:
> I would like to have the colorbar displaying the same color scaling
> regardless of the data input.

This /should/ be feasible with .contourf(.., vmin=VMIN, vmax=VMAX),
but from the doc this isn't clear at least.  If it doesn't work, we
have to work out how to use the norm=... argument properly to hand
over a Norm instance.  Please give vmin/max a try, we can look that up
if it fails.

> Problem 2: If I have a colorbar scale which goes from -10 to 5, and I use
> one of the standard colormaps which has white in the center of it. How do I
> keep the zero point at the white color instead of the colorbar automatically
> picking the midpoint for zero?

So you want to set the color-MAP's range with the solution to problem
1, and display in the color-BAR only a subset of the value range used
by the Norm?  This has been solved already in a former task, if it
suits you I can send the sources for that.

Friedrich

On 08/11/2010 06:13 AM, tgabriel wrote:
>
>
> Problem 1: I have a dataset which has more values on one side of the scale
> than the other in a contour map. Matplotlib will see that I have more values
> towards one side of the scale, and put all of the colors in that color map
> to that side of the scale.
>
> For example, lets say I would like my colorbar to show values from -10^6 all
> the way to 10^6.
> But, most of my values are in the -10^6 to -10^5 range. How do I get my
> colorbar not to automatically send all of the colors to that portion of the
> colorbar and leave everything greater than -10^5 to have one color.
>
> I would like to have the colorbar displaying the same color scaling
> regardless of the data input.
>
> Here is a copy of my code:
> v2 =
> [-1000000,-100000,-10000,-1000,-100,-10,0,10,100,1000,10000,100000,1000000]
> 	plt.subplot(6,1,2)
> 	cset4 = plt.contourf(x2,z2,k2,v2,cmap=cm.get_cmap('jet', len(v2)-1))
> 	cset5 = plt.contour(x2,z2,k2,v2,colors = 'k',linewidths = .22,hold='on')
> 	#set axes values
> 	plt.axis([xmin2,xmax2, zmin,zmax])
> 	#set y axes label
> 	plt.ylabel('Z, [km]')
> 	#display title
> 	plt.title('v3 Plot              Max:'+str("%.2f" %k2.max())+'		   Min:
> '+str("%.2f" %k2.min()),size='small')
> 	#colorbar manipulation
> 	cbar = plt.colorbar(cset4,spacing="uniform", shrink =
> 0.95,ticks=[-1000000,-100000,-10000,-1000,-100,-10,0,10,100,1000,10000,100000,1000000])
>
> cbar.ax.set_yticklabels(['-10^5','-10^4','-10^3','-10^2','-10^1',0,'10^1','10^2','10^3','10^4','10^5'])
> 	#close file
> 	f2.close()

I don't understand the problem based on your description and code. 
Please make a stand-alone example script, runnable by anyone, generating 
fake data as needed in the script.  It should require only a little 
reworking of the fragment that you included above.  Please include the 
script as an attachment, so it does not get mangled by the mail sending 
or reading software.

Also, please specify what mpl version you are using.

Eric
>
> Problem 2: If I have a colorbar scale which goes from -10 to 5, and I use
> one of the standard colormaps which has white in the center of it. How do I
> keep the zero point at the white color instead of the colorbar automatically
> picking the midpoint for zero?
>
> Problem 1 is much more of a pressing issue right now than problem 2.
>
> Thank you
>

On 08/11/2010 08:54 AM, Friedrich Romstedt wrote:
> 2010/8/11 tgabriel<tra...@gm...>:
>> I would like to have the colorbar displaying the same color scaling
>> regardless of the data input.
>
> This /should/ be feasible with .contourf(.., vmin=VMIN, vmax=VMAX),
> but from the doc this isn't clear at least.  If it doesn't work, we
> have to work out how to use the norm=... argument properly to hand
> over a Norm instance.  Please give vmin/max a try, we can look that up
> if it fails.

No, contour* do not use vmin, vmax kwargs. Setting the levels should be 
enough for what I thought the OP wanted to do, so I don't understand the 
problem--hence I requested a standalone script to illustrate it.

vmin, vmax can still be set by using the plt.clim function.

Maybe the OP's problem is that the colors become indistinguishable in 
the relatively small intervals?  If so, the answer is to use a different 
norm, or to use a ListedColormap with a color for each interval.  The 
ListedColormap allows complete freedom to explicitly specify the color 
for each interval between contour boundaries.

>
>> Problem 2: If I have a colorbar scale which goes from -10 to 5, and I use
>> one of the standard colormaps which has white in the center of it. How do I
>> keep the zero point at the white color instead of the colorbar automatically
>> picking the midpoint for zero?
>
> So you want to set the color-MAP's range with the solution to problem
> 1, and display in the color-BAR only a subset of the value range used
> by the Norm?  This has been solved already in a former task, if it
> suits you I can send the sources for that.

This sounds like a job for a custom norm, but maybe you found a better way?

Eric

>
> Friedrich
>
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>
> Maybe the OP's problem is that the colors become indistinguishable in
> the relatively small intervals?  If so, the answer is to use a different
> norm, or to use a ListedColormap with a color for each interval.  The
> ListedColormap allows complete freedom to explicitly specify the color
> for each interval between contour boundaries.

See 
http://matplotlib.sourceforge.net/examples/pylab_examples/contourf_demo.html