There are two ways you can set the x-axis category labels for the chart. One way is to put your labels into a range on your worksheet and then include them in the data source for your chart.
Right-click on the waterfall chart and go to Select Data. Add a new series using cell I4 as the series name, I5 to I11 as the series values and C5 to C11 as the horizontal axis labels. Right-click on the waterfall chart and select Change Chart Type. Change chart type of the data label position series to Scatter. Here's one way: Select only the Y numerical data. Choose Insert > Chart > Line. With the chart selected, in the Chart Design ribbon click Select Data. In the Select Data Source dialog, click in the Horizontal (Category) axis labels edit box, and select the X text (or numerical) values on the worksheet.
This is usually done when you create the chart. If the labels are entered in the row above the chart data, Excel will recognize them as x-axis labels. The graphic below shows an example of the setup. Then you would select the entire range with both labels & data-cells C8:E11-in this case, and insert your chart, with results as shown. This method works with all versions of Excel.
The second way involves editing the chart's settings after it has been created. Step-by-step, here's what's involved (This may work in older versions, but I did it in Excel 2010). Select your chart and then pick the Chart Tools/Design options on the ribbon. Then choose Select Data from the options (still on the ribbon). The chart data range is shown at the top of the dialog box that comes up, with a box below left to make changes to the chart settings for the data series and a box at below right to make changes to the x-axis labels.
The latter box will list the '1', '2', etc. Numbers that you want to change. Hit the edit button for the right-hand box (Horizontal Category (Axis) Labels), and you will be prompted to enter an axis label range.
Instead of selecting a range, though, just enter the labels that you want to see on the x-axis, separated by commas, like so:. Press OK, and then again when the Select Data Source dialogue reappears, and it's done.