use metric string, not metric object, to generate remove url
fix padding issues with scaled y axis and stacked graphs

this should correct issues with 0 delta, small ratechange and larger change graphs.
rickshaw.js appears to have different ways it handles scaled max/min, and appears to prefer scalar changes to minimums and ratio (explicit padding) changes to maximum. Setting one or the other doesn't effect both, so it now uses both methods to set a larger padding around smaller delta graphs. Should rickshaw change, this may break