Last updated on 2023-06-08 | Edit this page
Estimated time 30 minutes
- How can I plot my data?
- How can I save my plot for publishing?
- Create a time series plot showing a single data set.
- Create a scatter plot showing relationship between two data sets.
matplotlib is the
most widely used scientific plotting library in Python.
- Commonly use a sub-library called
- The Jupyter Notebook will render plots inline by default.
import matplotlib.pyplot as plt
- Simple plots are then (fairly) simple to create.
= [0, 1, 2, 3] time = [0, 100, 200, 300] position plt.plot(time, position)'Time (hr)') plt.xlabel('Position (km)')plt.ylabel(
In our Jupyter Notebook example, running the cell should generate the figure directly below the code. The figure is also included in the Notebook document for future viewing. However, other Python environments like an interactive Python session started from a terminal or a Python script executed via the command line require an additional command to display the figure.
matplotlib to show a figure:
This command can also be used within a Notebook - for instance, to display multiple figures if several are created by a single cell.
Plot data directly from a
- We can also plot Pandas dataframes.
- Before plotting, we convert the column headings from a
integerdata type, since they represent numerical values, using str.replace() to remove the
gpdPercap_prefix and then astype(int) to convert the series of string values (
['1952', '1957', ..., '2007']) to a series of integers:
[1925, 1957, ..., 2007].
import pandas as pd = pd.read_csv('data/gapminder_gdp_oceania.csv', index_col='country') data # Extract year from last 4 characters of each column name # The current column names are structured as 'gdpPercap_(year)', # so we want to keep the (year) part only for clarity when plotting GDP vs. years # To do this we use replace(), which removes from the string the characters stated in the argument # This method works on strings, so we use replace() from Pandas Series.str vectorized string functions = data.columns.str.replace('gdpPercap_', '') years # Convert year values to integers, saving results back to dataframe = years.astype(int) data.columns 'Australia'].plot()data.loc[
- By default,
DataFrame.plotplots with the rows as the X axis.
- We can transpose the data in order to plot multiple series.
data.T.plot()'GDP per capita')plt.ylabel(
- For example, do a bar plot using a fancier style.
'ggplot') plt.style.use(='bar') data.T.plot(kind'GDP per capita')plt.ylabel(
- The command is
- The color and format of markers can also be specified as an
additional optional argument e.g.,
b-is a blue line,
g--is a green dashed line.
= data.columns years = data.loc['Australia'] gdp_australia 'g--')plt.plot(years, gdp_australia,
# Select two countries' worth of data. = data.loc['Australia'] gdp_australia = data.loc['New Zealand'] gdp_nz # Plot with differently-colored markers. 'b-', label='Australia') plt.plot(years, gdp_australia, 'g-', label='New Zealand') plt.plot(years, gdp_nz, # Create legend. ='upper left') plt.legend(loc'Year') plt.xlabel('GDP per capita ($)')plt.ylabel(
Often when plotting multiple datasets on the same figure it is desirable to have a legend describing the data.
This can be done in
matplotlib in two stages:
- Provide a label for each dataset in the figure:
='Australia') plt.plot(years, gdp_australia, label='New Zealand')plt.plot(years, gdp_nz, label
matplotlibto create the legend.
By default matplotlib will attempt to place the legend in a suitable
position. If you would rather specify a position this can be done with
loc= argument, e.g to place the legend in the upper
left corner of the plot, specify
- Plot a scatter plot correlating the GDP of Australia and New Zealand
- Use either
= 'Australia', y = 'New Zealand')data.T.plot.scatter(x
Fill in the blanks below to plot the minimum GDP per capita over time for all the countries in Europe. Modify it again to plot the maximum GDP per capita over time for Europe.
= pd.read_csv('data/gapminder_gdp_europe.csv', index_col='country') data_europe ='min') data_europe.____.plot(label data_europe.____='best') plt.legend(loc=90)plt.xticks(rotation
= pd.read_csv('data/gapminder_gdp_europe.csv', index_col='country') data_europe min().plot(label='min') data_europe.max().plot(label='max') data_europe.='best') plt.legend(loc=90)plt.xticks(rotation
= pd.read_csv('data/gapminder_gdp_asia.csv', index_col='country') data_asia ='scatter', x='min', y='max')data_asia.describe().T.plot(kind
No particular correlations can be seen between the minimum and maximum gdp values year on year. It seems the fortunes of asian countries do not rise and fall together.
You might note that the variability in the maximum is much higher than that of the minimum. Take a look at the maximum and the max indexes:
= pd.read_csv('data/gapminder_gdp_asia.csv', index_col='country') data_asia max().plot() data_asia.print(data_asia.idxmax()) print(data_asia.idxmin())
Seems the variability in this value is due to a sharp drop after 1972. Some geopolitics at play perhaps? Given the dominance of oil producing countries, maybe the Brent crude index would make an interesting comparison? Whilst Myanmar consistently has the lowest gdp, the highest gdb nation has varied more notably.
This short program creates a plot showing the correlation between GDP and life expectancy for 2007, normalizing marker size by population:
= pd.read_csv('data/gapminder_all.csv', index_col='country') data_all ='scatter', x='gdpPercap_2007', y='lifeExp_2007', data_all.plot(kind=data_all['pop_2007']/1e6) s
Using online help and other resources, explain what each argument to
A good place to look is the documentation for the plot function - help(data_all.plot).
kind - As seen already this determines the kind of plot to be drawn.
x and y - A column name or index that determines what data will be placed on the x and y axes of the plot
s - Details for this can be found in the documentation of plt.scatter. A single number or one value for each data point. Determines the size of the plotted points.
If you are satisfied with the plot you see you may want to save it to a file, perhaps to include it in a publication. There is a function in the matplotlib.pyplot module that accomplishes this: savefig. Calling this function, e.g. with
will save the current figure to the file
The file format will automatically be deduced from the file name
extension (other formats are pdf, ps, eps and svg).
Note that functions in
plt refer to a global figure
variable and after a figure has been displayed to the screen (e.g. with
plt.show) matplotlib will make this variable refer to a new
empty figure. Therefore, make sure you call
before the plot is displayed to the screen, otherwise you may find a
file with an empty plot.
When using dataframes, data is often generated and plotted to screen
in one line. In addition to using
plt.savefig, we can save
a reference to the current figure in a local variable (with
plt.gcf) and call the
savefig class method
from that variable to save the figure to file.
='bar') data.plot(kind= plt.gcf() # get current figure fig 'my_figure.png')fig.savefig(
Whenever you are generating plots to go into a paper or a presentation, there are a few things you can do to make sure that everyone can understand your plots.
- Always make sure your text is large enough to read. Use the
labelsizeto increase the text size of the numbers on your axes.
- Similarly, you should make your graph elements easy to see. Use
sto increase the size of your scatterplot markers and
linewidthto increase the sizes of your plot lines.
- Using color (and nothing else) to distinguish between different plot
elements will make your plots unreadable to anyone who is colorblind, or
who happens to have a black-and-white office printer. For lines, the
linestyleparameter lets you use different types of lines. For scatterplots,
markerlets you change the shape of your points. If you’re unsure about your colors, you can use Coblis or Color Oracle to simulate what your plots would look like to those with colorblindness.