Specify the default line types for groups in graphs

File > Options > Graphics > Data View With Groups > Line Types

For graphs that have lines, you can assign a different line type to each group. Specify the default number and types of lines that are available. The changes you make to the defaults remain until you change them again, even after you exit Minitab.

If the number of groups in the data is higher than the number of available line types, then Minitab repeats the line types.