Graph wrong value assigned by excel to horizontal axis

Hello Sara,

how Excel determines which and how columns are charted when inserting a chart is dependant on how you have selected the data. If the result does not suit (what can happen), you can change and adapt your chart at any time.

So, in your chart, delete the month series on the bottom by clicking on it and e.g. pressing the Del key. Then right click the chart and select the option Select data. In the dialogue, on the right side, click on Edit to assign a range ($C$3:$C$40) to the horizontal axis labels.

Regarding the second question, I don't understand why changing the month order produces duplicates. May be you can explain this in more detail. For sorting, I selected the range and then used Home / Sort & Filter / Sort Largest to smallest. I attached your file with my changed here.

Best,

Mourad