All callers of create_plot_info_new() called calculate_max_limits_new()
a line before. Thus, simply call the latter in the former.
This allows us to automatically free the plot data in create_plot_info_new().
The old code overwrote the corresponding field with NULL.
As a side-effect, this removes a bogus static variable.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>