Question

I have two columns of data, x and y. The y data takes the shape of the triangle wave below. As you can see, the triangle has 2 sections of positive gradient and 1 longer section with a negative gradient.

I would like to write a program that:

• Queries whether the current entry in an vertical array has a positive or negative gradient with respect to the successive entry in the array.
• Then, plots the y data against x, where y values with a positive gradient (and its respective x value) are plotted using one colour, and the negative points in another colour.

How is this best done in Python?

Answer 1

filen = 'filename.txt'
x = loadtxt(fn,unpack=True,usecols=[0]) 
y = loadtxt(fn,unpack=True,usecols=[1])

n = ma.masked_where(gradient(y) < 0, y)
p = ma.masked_where(gradient(y) > 0, y)

pylab.plot(x,n,'r',x,p,'g')

Does the trick for me!