Excel Chart Series: column has data, but dialog box shows zeros

Excel 2013. My machine is a 64-bit running Windows 7.

I have an .xslm file with a chart. The chart is created from a few columns of data. The data in the columns is populated by reading data from a bunch of other excel files. This is a 2-D line chart with the horizontal axis as dates (in text) and the vertical axis as hours (expressed as h:mm) with the actual values between 541:50 and 757:00. For most of the cases, it works fine. However, for 3 cases, My graph flatlines at zero. I look at the column data (column G, representing Cumulative Hours) – it is correct! When I edit the series data, I can see that it correctly has Series values = "$G$2:$G$100".

That's fine, however the edit series dialog box also shows the values of those cells and the values are all zeros (which is what is graphed) even though I can see the contents of the cells are not zero.

Next bit of weirdness. I have another graph of another column. It shows the graph of the next column (H, which is derived from G), but it is correct. It doesn't flatline.

^{(Click image to enlarge)}

Note:

1. The column G2:G100 does have data.
2. The chart is flatlined.
3. The dialog box for the data series shows the correct range G2:G100, but
4. The values to the right of the dialog say 0.00, 0.00, …