If you are displaying the dataframe, df, from the first example that works, you can see that
“total_bill” and “tip” are column names. Hence your new example should be:
Plot(dftips, x= :total_bill, y=:tip, xbingroyp="x", ybingroup="y", kind="histogram2d",
facet_row=:sex, facet_col=:smoker)
because perhaps the underlying low level code will take for x, y, facet_row and facet_col the corresponding column names from your passed dataframe, dftips.