BarChart: Add support for x label per field mode

Graph (old) panel has an x axis mode called Series, which is pretty useful and feels common and simple conceptually for users. we should add handling/support for this frame shape if we dont detect any string or time fields to use for x. this would be helpful when auto-migrating Graph to BarChart. we could instead consider adding a color mode to BarGauge that that colors bars by field color config (or series index) instead of uniform or threshold.