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Bar and Horizontal Bar Chart
The Bar Chart displays data of one or more series as bars:

Bar Chart
By changing the chart typ to Horizontal Bar Chart the chart changes to the specified type. No other changes, besides optionally changing the Number Formatter Y-Axis settings to the Number Formatter X-Axis settings and vice versa:

Horizontal Bar Chart
The Bar Chart expects data in a two (or more) column format such as:
Date | Revenue | Cost |
23 January | 38 | 24 |
24 January | 39 | 24 |
25 January | 40 | 24 |
26 January | 40 | 24 |
In the Graph Designer you can then select the column you want to use for your x-axis (for the Horizontal Bar Chart the selected x-axis will be the y-axis):

Graph Designer 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 bar will be displayed.