You can use these additional steps to get the linear fit model and calculate its values, and then plot them.
fit_model = polyfit(T.visitors,T.sales,1);
TSales_Fit = polyval(fit_model,T.visitors);
Note that your set x -axis is T.date and what you are trying to plot on top of that is T.visitors (x-axis) vs. Tsales_Fit (y-axis). There is a mismatch. Thus, it is more appropriate to compute fit model: DD=datenum(T.date); TSales_Fit = a + b*(DD-DD(1)+1); and plot the followings:
fit_model = polyfit(DD,T.sales,1);
TSales_Fit = polyval(fit_model,DD);
legend('Date vs. Visitors', 'Date vs. Sales', 'Fit model: Date vs. Sales', 'location', 'best')