Have you ever wanted to draw a graph to visualize data in Python?

Python has an excellent library for creating graphs called Matplotlib

This article describes how to use the most common Axes.plot function for line and scatter plots.

It also explains how to customize colors, line types, thickness, etc.

Line graphs (Axes.plot)

The most basic graphs in Matplotlib use the Axes.plot function

Entering only y in Axes.plot displays y values at equal intervals on the graph, but entering both x and y results in an x versus y graph.

The following tabs explain the code and flowchart

# step0 Import libraries
import matplotlib.pyplot as plt
import numpy as np

# step1 Crete data
x = np.linspace(0, 10, 100)
y1 = 4 + 2 * np.sin(2 * x)
y2 = 4 + 2 * np.cos(2 * x)

# step2 Create data frames
fig, ax = plt.subplots()

# step3 Plot line graphs
ax.plot(x, y1, linestyle='-', label='Sample 1')
ax.plot(x, y2, linestyle='--', label='Sample 2')

# step4 Set the axis, legend, title
ax.set_xlim(0, 8)
ax.set_ylim(0, 8)
ax.set_xlabel('X label')
ax.set_ylabel('Y label')
ax.legend()
ax.set_title('Simple line')

# step5 Call the Figure
plt.show()

Scatter plot (fmt)

Scatter plots with the Axes.plot function do not specify line styles, only markers.

In this case, it is convenient to use fmt (format string) to specify marker, line color, and line type at once.

or--: o=circle, r=red, --=dashed

# step3 Plot scatter plots
ax.plot(x, y1, 'o',label='Sample 1')
ax.plot(x, y2, 'or--', label='Sample 2')

Line style and transparency for line graphs

Line customization, such as color, type, and thickness, is done using color, linestyle, and linewidth

alpha=0~1 changes the transparency.

Line color (color)

For line color, enter color as the argument.

color='red': red

color='00BCD4': color code

# step3 Plot line graphs
ax.plot(x, y1, label='red', color='red')
ax.plot(x, y2, label='#00BCD4', color='#00BCD4')

Line type (linestyle)

Use linestyle for the line type.

linestyle='-': solid
linestyle='--': dashed
linestyle='-.': dashdot
linestyle=':': dotted
linestyle='': None

# step3 Plot line graphs
ax.plot(x, y1, label='dashed', linestyle='-.')
ax.plot(x, y2, label='dotted', linestyle=':')

Line width and transparency (linewidth, alpha)

For line thickness, enter a numerical value in linewidth.

linewidth=5: Thickness 5

For line transparency, enter alpha=0.5 and a number between 0~1

# step3 Plot line graphs
ax.plot(x, y1, label='linewidth=5', linewidth=5)
ax.plot(x, y2, label='lw=5, alpha=0.5', linewidth=5, alpha=0.5)

Markers for line graphs (marker)

The marker type can be selected by entering a string from matplotlib.markers

only markers with fmt: no line = scatter plot
marker with marker: with line = line chart

Specify markers with marker

Markers are circles, squares, and triangles

marker='o': circle, marker='s': square, marker='^': triangle

# step3 Plot line graphs
ax.plot(x, y1, marker='o', label='marker=o')
ax.plot(x, y2, marker='s', label='marker=s')
ax.plot(x, y2-2, marker='^', label='marker=^')

Specify only markers with fmt

fmt (format string) specifies marker, line color, and line type at once

Details are shown in the scatter plot

# step3 Plot sctter plots
ax.plot(x, y1, 'o', label='fmt=o')
ax.plot(x, y2, 's', label='fmt=s')
ax.plot(x, y2-2, '^', label='fmt=^')

Marker color for line graphs

Marker color can be divided into two types: surface and border.

The area to be filled can also be specified.

Main color of the marker (markerfacecolor)

Marker surface colors were set to white and black

markerfacecolor='white': White surface

# step3 Plot line graphs
ax.plot(x, y1, marker='o', label='face=white', markerfacecolor='white')
ax.plot(x, y2, marker='s', label='face=black', markerfacecolor='black')

Border color and width of the marker (markeredgecolor, markeredgewidth)

The color and thickness of the marker’s border can be changed.

markeredgecolor='red': Red border
markeredgewidth=3: Thickness 3 border

# step3 Plot line graphs
ax.plot(x, y1, marker='o', label='edgecolor=red', 
        markeredgecolor='red')
ax.plot(x, y2, marker='s', label='edgewidth=3', 
        markeredgecolor='red', markeredgewidth=3)

Fill area of the marker (fillstyle)

The fill area of the marker is determined by the fillstyle

The available areas are full, left, right, bottom, top, none.

# step3 Plot line graphs
ax.plot(x, y1, marker='o', label='fill=top', fillstyle='top')
ax.plot(x, y2, marker='s', label='fill=bottom', fillstyle='bottom')

Clean format for thesis

Combining the two marker types + surface color will produce a beautiful graph.

markerfacecolor='white': White surface
marker='o': circle, marker='s': square, marker='^': triangle

# step1 Create data
x = np.linspace(0, 10, 100)
y1 = 4 + 2 * np.sin(2 * x)
y2 = 4 + 2 * np.cos(2 * x)

# step2 Create graph frames
fig, ax = plt.subplots()

# step3 Plot line graphs
ax.plot(x, y1, marker='o', label='marker=o', markerfacecolor='white')
ax.plot(x, y2, marker='s', label='marker=s', markerfacecolor='white')
ax.plot(x, y2-2, marker='^', label='marker=^', markerfacecolor='white')
  
# step4 Set the axis, legend, title
ax.set_xlim(0, 8)
ax.set_ylim(0, 8)
ax.set_xlabel('X label')
ax.set_ylabel('Y label')
ax.legend()
ax.set_title('Plot graph for a thesis')

# step5 Call the Figure
plt.show()

References

Axes.plot Function

lines.Line2D class