Add various missing options to Grapher

This patch adds following missing options to grapher/fixes a typo (Legand -> Legend) and formats README.md

-a (imageformat [string])
Image format [PNG | SVG | EPS | PDF] for the generated graph.

-i (interlaced [bool])

If images are interlaced they become visible on browsers more quickly.

-b (base [value])

If you are graphing memory (and NOT network traffic) this switch should be set to 1024 so that one Kb is 1024 byte. For traffic measurement, 1 kb/s is 1000 b/s.

-W (watermark [string])

Adds the given string as a watermark, horizontally centered, at the bottom of the graph.