bb-matrix-plot: Use interpolation for sparse data

bb-matrix-plot: Use interpolation for sparse data

If not every combination of BB_NUMBER_THREADS and PARALLEL_MAKE have
been tested by bb-matrix.sh, e.g., by using BB_RANGE="04 08 10 12 16"
and PM_RANGE="04 08 10 12 16", then the graph that gnuplot generates by
default looks very jagged due to the missing data points. By using
splines to interpolate the missing data the graph looks a lot better.

This should not change graphs where all data points are available in any
way, only improve sparse graphs.

(From OE-Core rev: 9642c1314da64c70254f6b012aa73ef37bbaa33f)

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>