Lead with the question
Before adding a chart, write the decision it should help someone make. A balance, a trend and an exception each need a different reading pattern. Let that question determine which number gets the most space.
A number needs a frame of reference.
Keep comparisons honest
Show the period, unit and scope beside the value. Avoid making a small change look dramatic through an unexplained axis or selective range. Use colour to identify a meaningful state, with a label that still works without it.
Design for uncertainty
An empty result is not the same as zero. Make loading, unavailable and no-data states distinct. A useful interface explains what is known and what the person can do next, instead of filling the space with a reassuring but misleading number.
