When you create a new chart in Excel, you must specify the data to be plotted (for more information please see How to Make a Line Graph in Microsoft Excel). When you create a line chart using one column of data Excel adds only one plot line to the chart. But when you include two or more columns of data, Excel treats each column as a separate data series and automatically creates separate lines on the chart for the data in each column.

If you already have a spreadsheet with data entered into columns, skip to the next step (Create Your Chart, below). In the first row of the worksheet, enter the labels for each of the data series to plot. Since each data series (the data for each separate line to be plotted) must be in a separate column, you must enter the labels for each data series in a separate column on the first row. Excel uses the labels in the first row of each column to label the lines in the chart for the data in that column. Enter your data into each of your columns, in the rows below the labels. Here’s an example worksheet showing two data series. The data for each series is in a separate column, and the label to display on the chart for each is in the first row of each column.

If you already have a line chart, you may add a new plot line to it by editing the Data Source for the chart. Enter or copy and paste the data for the new plot line into the column immediately to the right of the original data. Make sure to add a label to the first row of the new column so the chart can display a label for the new plot line. Click on the chart that displays the original data to select it. Right-click on the chart and choose Select Data… from the pop-up menu. Alternately, click Select Data from the Data section of the Office Ribbon in the Chart Tools Design tab. The Select Data Source dialog opens. The data displayed in the chart is shown in the Chart data range field. 4. In the Chart data range field, change the last letter of the displayed range, to make it match the letter of the last column of the new data. In the example, the Chart data range displays =Sheet2!$A$1:$A$12, but the new column of data is column $B. So the last A (in the cell reference $A$12) must be changed to a B. The resulting Chart data range displays =Sheet2!$A$1:$B$12. After changing the Chart data range, press the OK button. The chart updates to display an additional plot line for the new data.