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How does one display the fit line in this plot in this situation?

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I have a timetable T with
T.date T.vistors T.sales
----------------------------------------
I draw a plot
yyaxis left
plot(T.date, T.visitors)
hold on
yyaxis right
plot(T.date, T.sales)
Now I want to add a regression line of T.sales = a + b*T.visitors on this plot.
But this situation seems to be more complex. How to proceed?

Accepted Answer

Sulaymon Eshkabilov
Sulaymon Eshkabilov on 21 Aug 2021
T = readtable('https://www.mathworks.com/matlabcentral/answers/uploaded_files/717514/test.csv');
Warning: The DATETIME data was created using format 'MM/dd/uuuu' but also matched 'dd/MM/uuuu'.
To avoid ambiguity, supply a datetime format using SETVAROPTS, e.g.
opts = setvaropts(opts,varname,'InputFormat','MM/dd/uuuu');
fit_model = polyfit(T.Visitor, T.Sales,1);
TSales_Fit = polyval(fit_model,T.Visitor);
yyaxis left
plot(T.Date, T.Visitor, 'ro', 'markerfacecolor', 'c', 'markersize', 13)
hold on
yyaxis right
plot(T.Date, T.Sales, 'bs', 'markerfacecolor', 'y', 'markersize', 9)
hold on
plot(0:numel(TSales_Fit)-1,TSales_Fit, 'k-', 'linewidth', 2)
legend('Date vs. Visitors', 'Date vs. Sales', 'Fit model: Visitors vs. Sales', 'location', 'northwest')
grid on

More Answers (1)

Sulaymon Eshkabilov
Sulaymon Eshkabilov on 20 Aug 2021
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);
yyaxis left
plot(T.date, T.visitors)
hold on
yyaxis right
plot(T.date, T.sales)
hold on
plot(T.visitors, TSales_Fit)
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:
DD=datenum(T.date);
DD = DD-DD(1)+1;
fit_model = polyfit(DD,T.sales,1);
TSales_Fit = polyval(fit_model,DD);
yyaxis left
plot(T.date, T.visitors)
hold on
yyaxis right
plot(T.date, T.sales)
hold on
plot(T.date, TSales_Fit)
legend('Date vs. Visitors', 'Date vs. Sales', 'Fit model: Date vs. Sales', 'location', 'best')
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