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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