benchplot rewritten

[jangorecki]

visible changes:

• question headers (question, more precise stats)
• consistent x/DF/DT naming
• handling of single solution
• number of ticks on X axis capped to more pretty
• better scaling for various number of solutions
• timing values might had sometimes ... prefix even when they fit the X lim plot, it is now fixed
• even if timings of cutoff solution (spark) doesn't exists plot will procedure without cutoff