Dashboards are extremely limited in how they can be used for directly comparing data captured in different custom fields. Currently only one custom field can be reported on in a bar chart or line chart, which mean if I want to report on 'Estimated Hours' vs. 'Hours worked' for tasks grouped by 'Work Type' drop down or something similar, it isn't possible.

If implemented, this could add a lot of value to these kinds of custom dashboard cards.