Use descriptive captions

The caption of a chart should describe what the chart is completely about. In a chart showing the website traffic for a particular time period, the caption should clearly mention the purpose of the chart along with the time period for which the data is being displayed, as shown in the following screenshot:

The caption should also mention the scale and the units of the numbers on the chart, if any. In fact, the caption should be so descriptive that the axes titles, if used, are only present as reinforcements.

However, with so many details in the caption, it might be difficult to accommodate everything in one line. That is when the subcaption comes into play. Typically, the units and scale of the numbers on the chart are delegated to the subcaption and so are details of lesser importance, as shown in the following screenshot: