The Line Chart displays data of one or more series as lines:
Expected Data and Graph Designer
The Line Chart expects data in a two (or more) column format such as:
In the Graph Designer you can then select the column you want to use for your x-axis:
Hence, the x-axis column does not need to be the first column. However, for consistency purposes it is recommended to always use your first column for the x-axis.
For each column, besides the one used by the X-axis one line will be displayed.