Scatter charts are often used for visualizing data with lots of elements or when the data intervals are inconsistent.
A scatter chart requires at least one X Axis and one Y Axis data. You can add more than one data to the Y Axis field to create multiple series.
Alternatively, you can select a Group to separate the Y Axis data accordingly.
Please note that the Y Axis needs to contain numeric fields and remember to select an Expression for each numeric Y Axis field by clicking on the field first and then choosing from the drop down.