How to plot data

When ploting in python we use the plot() function part of the matplotlib library. In the plot function you give the X and Y parameter. Remember that X and Y need to have the same length. using plt.title, plt.xlabel and plt.ylabel you define a title, x and y labels. In this class as well as outside of this class, for your thesis or reports, a plot always needs axes labels and ideally a title and a legend. You need the plt.show() comment at the end, which tells python to plot the data in the format you specified before:

# Import both modules:
import matplotlib.pyplot as plt
import numpy as np

# E.g. salinity vs temperature:
array2D=np.array([[5.2, 3.0, 4.5], [9.1, 0.1, 0.3]])

# Plotting point data, make sure that both vectors you want to plot have the same length:
temp=array2D[0, :]
sal=array2D[1, :]

plt.plot(temp, sal, "o")
plt.title("Salinity vs Temp in the Atlantic")
plt.xlabel("temperature")
plt.ylabel("salinity")
plt.grid()

plt.show()

The default of the plot() function is to draw a line. The style of the line can be changed using:

plt.plot(x, y, linestyle=":", color="b") # Dotted blue line
plt.plot(x, y, linestyle="-.", color="y") # Dash-Dot yellow line
plt.plot(x, y, linestyle="--", color="g") # Dashed green line

You can read more about line styles here: https://matplotlib.org/stable/gallery/lines_bars_and_markers/linestyles.html

Exporting plots

In order to save a plot you are creating in python simply use the plt.savefig("name.png") for a png-file or plt.savefig("name.pdf") for a pdf file:

import matplotlib.pyplot as plt

# Setting the size of the figure -important in case you want this to fit perfectly your page
fig, ax = plt.subplots(figsize=(10, 6))

x = [1, 2, 3, 4, 5, 6]
y = [0, 2, 4, 6, 8, 10]

# Plotting the data
ax.plot(x, y)

ax.set_title("Title", fontsize=20)
ax.set_xlabel("Time", fontsize=16)
ax.set_ylabel("Intensity", fontsize=16)
plt.savefig("my_plot.png")