Add update field to chart widget. {dm}, {dh} etc.

Currently the chart widget apparently only updates when a dataset changes. The new field would default to current behavior but offer the option of an update interval unrelated to a dataset minor change like {ds}, {dm}, {dh}, etc.
I kludged it by adding a fake dataset {dh} & masking it out, to plot hourly battery level. Not the best but it works.