How to explain excel what it have to use on chart?

I have a form that builds a table in a worksheet, and then make a chart, a simple chart with just one series.
However, excel doesn't know what it should put in X and Y. For example:

13X --- 10
14X --- 11
ABC --- 15
The chart's ok. It writes 13X, 14X and ABC in the x axis and we have 3 columns: 10, 11 and 15.

13X --- 10
14X --- 11
123 --- 15
The chart's not ok. It makes two series (1 and 2) and puts just numbers in the x axis: 1, 2,3...
I tied to convert 1st column in "words", but it didn't solve.