Plotting a "signals" series on a Matplotlib chart

Question

I'm making a tool for chart analysis and I have the following problem. So far I have a chart which is made by the following code:

def makeTheChart2(self, ser1, ser2, ser3):
    ecchart.figure(2)
    ecchart.subplot(111)
    ecchart.plot(ser1,label = "Upper Band", color = "black")
    ecchart.plot(ser2, label = "Lower Band", color = "blue")
    ecchart.plot(ser3, label = "Price", color = "red")
    ecchart.legend(loc='upper center', bbox_to_anchor=(0.5, -0.05),
      fancybox=True, shadow=True, ncol=5)
    ecchart.ylabel('Indicators evolution')
    ecchart.suptitle('Indicators', fontsize = 20)

The result is a chart showing a red line (which is a stock price) contained between an upper (black) and a lower (blue) band (sorry I cannot post the image, I'm new to Stack Overflow so I've not enough reputation yet).

The three series that are plotted are "ser1", "ser2" and "ser3". Now, assume that I have a fourth series which is not made of float numbers, but of booleans "True and False". Specifically, the list will be "True" when the red line is crossing either the black or the blue line, "False" viceversa. Is there a way to "plot" this information, or better to add a label to the chart everytime the red line "Price" is touching/crossing one of the other two? (I guess this info would be contained into the fourth list, something like displaying a small arrow everytime the list value is True).

Thanks in advance.

Answer 1

Have a look at this answer: matplotlib: Set markers for individual points on a line

To obtain your markers_on (as it is called in the referenced answer) you could do the following:

>>> import numpy as np
>>> x = np.array([1,2,3,4,5])
>>> b = np.array([True,False,False,True,False]) # this is your boolean data array
>>> markers_on = x[~b]
>>> markers_on
array([2, 3, 5])