Matplotlib - connecting lines using ma.masked.where methods

Question

So I am using numpy.ma.masked methods to plot lines under certain condition, but I would like to connect all the consecutive lines. For example, using this code:

import pylab as plt
import numpy as np
x = np.linspace(0,10,100)
y = -1.0 + 0.2*x
plt.plot(x,np.ma.masked_greater_equal(y,0))
plt.plot(x,np.ma.masked_less_equal(y,0),'r')

I get the following result: So what is the smart way of connecting the lines so have a continuous line that changes its color?

Answer 1

Look at your values of y. It looks like this:

array([-1.        , -0.97979798, -0.95959596, -0.93939394, -0.91919192,
       -0.8989899 , -0.87878788, -0.85858586, -0.83838384, -0.81818182,
       -0.7979798 , -0.77777778, -0.75757576, -0.73737374, -0.71717172,
       -0.6969697 , -0.67676768, -0.65656566, -0.63636364, -0.61616162,
       -0.5959596 , -0.57575758, -0.55555556, -0.53535354, -0.51515152,
       -0.49494949, -0.47474747, -0.45454545, -0.43434343, -0.41414141,
       -0.39393939, -0.37373737, -0.35353535, -0.33333333, -0.31313131,
       -0.29292929, -0.27272727, -0.25252525, -0.23232323, -0.21212121,
       -0.19191919, -0.17171717, -0.15151515, -0.13131313, -0.11111111,
       -0.09090909, -0.07070707, -0.05050505, -0.03030303, -0.01010101,
        0.01010101,  0.03030303,  0.05050505,  0.07070707,  0.09090909,
        0.11111111,  0.13131313,  0.15151515,  0.17171717,  0.19191919,
        0.21212121,  0.23232323,  0.25252525,  0.27272727,  0.29292929,
        0.31313131,  0.33333333,  0.35353535,  0.37373737,  0.39393939,
        0.41414141,  0.43434343,  0.45454545,  0.47474747,  0.49494949,
        0.51515152,  0.53535354,  0.55555556,  0.57575758,  0.5959596 ,
        0.61616162,  0.63636364,  0.65656566,  0.67676768,  0.6969697 ,
        0.71717172,  0.73737374,  0.75757576,  0.77777778,  0.7979798 ,
        0.81818182,  0.83838384,  0.85858586,  0.87878788,  0.8989899 ,
        0.91919192,  0.93939394,  0.95959596,  0.97979798,  1.        ])

You'll notice that there is no value of 0.0, thus the two lines will never touch.

You can resolve this by adding one more value to your x array (i.e. 101 values, so you have a spacing of 0.1, instead of 0.10101. You also need to remove the _equal from the masks, otherwise they will never touch (you currently mask out the value at y=0. in both cases).

import pylab as plt
import numpy as np
x = np.linspace(0.,10.,101)
y = -1.0 + 0.2*x
plt.plot(x,np.ma.masked_greater(y,0.))
plt.plot(x,np.ma.masked_less(y,0.),'r')