The tests for the plotting methods

• Column.boxplot
• Column.histogram
• Table.correlation_heatmap
• Table.lineplot
• Table.scatterplot

are insufficient. Now that the plotting methods return an Image (#159), we can test them more thoroughly:

1. Write test code that calls the plotting method on some simple data.
2. Store the output in a PNG file in tests/resources/image.
3. Put the test code into a pytest function to compute the current output of the plotting method.
4. Load the image file to get the expected output.
5. Assert that the images are equal.

This way we can easily detect regressions, e.g. when we update dependencies.