Delegate detailed info to tooltips

Capabilities such as tooltips and drill-down are what make interactive charts much more useful than Excel-type images. In an image, all the information associated with a chart has to go on the chart itself. Quite often, this results in the chart getting cluttered with excessive details. to prevent that, the details are crimped. However, with interactive charts, all the detailed information can go in the tooltip. So in a chart showing daily footfall at a store, the explanation for why there was a sudden increase or decrease in the number of visitors can easily go in the tooltip, as shown in the following screenshot:

By putting the details in the tooltips, the chart stays clean and the user has all the required details at his disposal. However, how does the user know that he can get more info by hovering over a data point, or clicking on tablets and smartphones? The points with more details on them can be highlighted and we can add a simple message in the...