The Marker by (or Show one marker for each) selector specifies the column or hierarchy containing the categories by which the markers should be split.

If you select (Row Number) here, then each row in the data table is shown as a separate marker in the visualization, even if several markers have identical values for the different axes, and therefore are located at the exact same coordinates. These markers will be drawn on top of each other.

However, you can select to combine markers (known as aggregation) so that each marker shows, for example, the sum of all rows that have something in common.

For example, if you have Continent on the X-axis and Sales on the Y-axis, then each individual sales number for each continent is shown as a marker:

However, if you change Marker by to Year (from the legend or using the visualization properties), then the sum of sales for each year will be shown as a marker for each continent:

If you rather would like to show, for example, the average, you can change which aggregation to use as on the axis.

In a map chart visualization, the selection you make on Marker by is always the same as the one used for geocoding/positioning the marker layer, so if you change the column or hierarchy here, it will be changed there as well. See Aggregating coordinate-based locations values for an example of how you can use the Marker by setting together with coordinate-based positioning in map charts.

You might also have specified other properties that contribute to a splitting or aggregation of markers.

For more information, see Slicing and marker concepts.