Matplotlib y axis values are not ordered

Question

I am trying to plot using matplotlib. The plot showed a problem that the Y axis is not ordered.

Here is the code.

# -*- coding: UTF-8 -*-
import matplotlib.pyplot as plt
import matplotlib.dates as mdates
import datetime
import numpy as np
I020 = [ line.strip('\n').split(",") for line in 
open(r'D:\Users\a0476\Anaconda3\TickData\PV5sdata1.csv')][1:]
Time = [ datetime.datetime.strptime(line[0],"%H%M%S%f") for line in I020 ]
Time1 = [ mdates.date2num(line) for line in Time ]
Solar = [ line[1] for line in I020 ]
order = np.argsort(Time1)
xs = np.array(Time1)[order]
ys = np.array(Solar)[order]
plt.title('Solar data')
plt.xlabel('Time')
plt.ylabel('Solar')
ax.plot_date(xs, ys, 'k-')
hfmt = mdates.DateFormatter('%H:%M:%S')
ax.xaxis.set_major_formatter(hfmt)
plt.show()

The CSV data

time        solar
7000000     50.35
8000000     41.01
9000000     69.16
10000000    94.5
11000000    111.9
12000000    103
13000000    98.6
14000000    36.45
15000000    34.74
16000000    34.17
17000000    34.6

Answer 1

The reason this happens is because your data is being plotted as strings.

The solution is to convert your y axis data to floats. This can be done by simply casting to a float in your list comprehension:

Solar = [float(line[1]) for line in I020]

I would also suggest to use matplotlib's auto formatting of the x -axis when using dates/times. This will rotate the labels etc to make the graph look better:

plt.gcf().autofmt_xdate()

Your example becomes:

I020 = [ line.strip('\n').split(",") for line in open('PV5sdata1.csv')][1:]
Time = [datetime.datetime.strptime(line[0],"%H%M%S%f") for line in I020]
Time1 = [mdates.date2num(line) for line in Time]
Solar = [float(line[1]) for line in I020]

xs = np.array(Time1)  # You don't really need to do this but I've left it in
ys = np.array(Solar)

fig, ax = plt.subplots() # using matplotlib's Object Oriented API

ax.set_title('Solar data')
ax.set_xlabel('Time')
ax.set_ylabel('Solar')
ax.plot_date(xs, ys, 'k-')

hfmt = mdates.DateFormatter('%H:%M:%S')
ax.xaxis.set_major_formatter(hfmt)
plt.gcf().autofmt_xdate()

plt.show()

Which gives: