Option -l for graphs

[Eike]

Implement option -l for graphs

Ideas:
- Plot lines/pale bands that denote upper/lower price limits.
    - 95% are below/above these limits. Implemented with generalized median operator, would also compute median. See: Numpy scoreatpercentile(a, per, limit = ())
    - Standard deviation.
- Histograms with progressively paler colors to denote the number of elements in the bins. Would result in pale double gradient roughly centered on median.