This method will guide you to manually add a data label from a cell of different column at a time in an Excel chart.

1. Right click the data series in the chart, and select Add Data Labels > Add Data Labels from the context menu to add data labels.

2. Click any data label to select all data labels, and then click the specified data label to select it only in the chart.

3. Go to the formula bar, type =, select the corresponding cell in the different column, and press the Enter key. See screenshot:

4. Repeat the above 2 - 3 steps to add data labels from the different column for other data points.

This method will introduce a solution to add all data labels from a different column in an Excel chart at the same time. Please do as follows:

1. Right click the data series in the chart, and select Add Data Labels > Add Data Labels from the context menu to add data labels.

2. Right click the data series, and select Format Data Labels from the context menu.

3. In the Format Data Labels pane, under Label Options tab, check the Value From Cells option, select the specified column in the popping out dialog, and click the OK button.

Now the cell values are added before original data labels in bulk.

4. Go ahead to untick the Y Value option (under the Label Options tab) in the Format Data Labels pane.

Note: If you are dealing with different types of charts, says clustered column chart, the option will display in another name, says Value option.
So far, you have added all data labels from the specified column in bulk.