Dropping results from the df might lead to df["application"].unique()
returning a different list of applications. The order of applications in
this list determines the order in which applications are plotted.

In order to respect the order of applications in the original DataFrame,
and thereby ensure that cascade and navchart plots produced from the
same projection use the same legend, we need to ensure that we do not
change the application order.

Signed-off-by: John Pennycook <[email protected]>

Plotting a NavChart requires a calculation of performance portability,
which may adjust the order of applications in the DataFrame.

Signed-off-by: John Pennycook <[email protected]>

The previous "expected" test result had actually been chosen based on
the empirical behavior of the library. If we expect the output of the
DataFrame to remain sorted by application, we should test for that.

Signed-off-by: John Pennycook <[email protected]>