A chart is a view of type report that has been flagged as a chart. A report can only be flagged as a chart when it is being created and once flagged as a chart it cannot be changed back to a normal report. The designer presents a specific layout when the report is flagged as a chart as displayed below

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The right hand side of the designer is split into three drop areas, one for each axis of the chart. You drop the category field into the x axis, the value field into the y axis and if you are designing a three dimensional chart, another category into the z axis. While you are designing the view there is no check regarding the data you drop in the axis areas, so you can configure the view incorrectly by dropping the category field into the y axis and the value field into the x axis. The same is true with the number of fields in the x and z axes: the chart view only supports one field in each axis but the designer does not check this, extra fields are simply ignored when the chart is drawn.