Plotting data from CSV files using matplotlib

Question

My csv file looks

and I want to draw the first column in x axis, second column as y axis. So my code is

import matplotlib.pyplot as plt
from numpy import genfromtxt
data=genfromtxt("test",names=['x','y'])
ax=plt.subplot(111)
ax.plot(data['x'],data['y'])
plt.show()

But this connect the end point of graph, showing straight line,
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What I want is this graph.
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Then how do I read data file or are there any options in matplotlib disconnecting the line?

If I type print data['x'] and print data['y'], it shows [ 0. 0.1 0.2 0.3 0.5 1. 0. 0.1 0.2 0.3 0.5 1. ] and [ 1. 2. 3. 4. 7. 9. 6. 10. 11. 12. 13. 14.] — user42298
– user42298, Commented May 7, 2015 at 3:12
@user42298 The above output doesn't look like output from a repr() of a list. Are you sure this is the output? Are you parsing the input values as floats? — James Mills
– James Mills, Commented May 7, 2015 at 3:14
Oh. Your x-values aren't in order, so the plot is going back to x=0 at the 6th point. Does that explain it? — cphlewis
– cphlewis, Commented May 7, 2015 at 3:21

Amen · Accepted Answer · 2015-05-07 04:40:32Z

1

As others mentioned in the comments, every call to plot will plot all the point-pairs it gets so you should slice the data for every column. If all the lines are of size 6 points you can do something like this:

import matplotlib.pyplot as plt
from numpy import genfromtxt
data=genfromtxt("test",names=['x','y'])
x=data['x']
y=data['y']
columnsize = int(len(x)/6)
ax=plt.subplot(111)
for i in range(columnsize):
    ax.plot(x[i*6:(i+1)*6],y[i*6:(i+1)*6])
plt.show()

this code works when x and y are of type numpy.ndarray. numpy arrays support indexing and slicing as python standard syntax.

edited May 7, 2015 at 4:40

answered May 7, 2015 at 4:16

Amen

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