Like the Bar chart, the line chart is one of the most versatile chart types. It is similar, but can indicate chronological data better. The line chart is good if, for example, you want to illustrate sales figures over a year (or any period of time). You'll then be able to easily see whether sales are going up, down or are stagnant. It will also become apparent which months are busier than others. By using a multiple line chart you can add multiple sets of data (eg two or more years of figures) to your chart. You can customise the line chart by stipulating it as being filled, having drop shadows, keys, having the X axis in the centre, different colors, different tickmark styles and tooltips etc.
The first graph uses external DIVs for the tooltip contents. The DIVs are at the bottom of this page (they're set not to be visible). Using external DIVs for tooltips can make maintenance easier.