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

[Eric F. <ef...@ha...>]

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




> 
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> 
> --Chad Kidder
> 
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