New external dependency ContourPy used for quad contour calculations#

Previously Matplotlib shipped its own C++ code for calculating the contours of quad grids. Now the external library ContourPy is used instead. There is a choice of four algorithms to use, controlled by the algorithm keyword argument to the functions contour and contourf. The default behaviour is to use algorithm='mpl2014' which is the same algorithm that Matplotlib has been using since 2014.

See the ContourPy documentation for further details of the different algorithms.


Contour lines and polygons produced by algorithm='mpl2014' will be the same as those produced before this change to within floating-point tolerance. The exception is for duplicate points, i.e. contours containing adjacent (x, y) points that are identical; previously the duplicate points were removed, now they are kept. Contours affected by this will produce the same visual output, but there will be a greater number of points in the contours.

The locations of contour labels obtained by using clabel may also be different.