In Japanese version, select a range by "X value" instead of "Index" when making a range selection for the plot, in the dialog that opens with "Data"("???" in Japanese)>"Edit Range"("?????") on the main menu. With the "By X"("X???") radio button selected, uncheck "Auto"("??"), an alert will appear if I try to enter a decimal point in "From"("??") or "To"("??").

Fixed in Origin 2021.