Upgraded to R2017a from R2016a and now fit function won't work

I'm getting the following error (the code worked fine with version R2016a). Can anyone help and explain why??

Error using cfit/plot (line 224)
Values plotted against x-axis must be datetime values. To create datetime values, use the DATETIME function.

Error in Well31 (line 17)
plot(f06,d2006,pf31.piezo_F31(r2006),'.b')

pf31.Date1 = datetime(pf31.date);

% 2006
figure
r2006 = 75:439;
plot(pf31.Date1(r2006),pf31.piezo_F31(r2006),'.b','DateTimeTickFormat','MMM-yy') % 2006 data
hold on
d2006=datenum(pf31.Date1(r2006));
f06 =  fit(d2006,pf31.piezo_F31(r2006),'poly3','Normalize','on','Robust','Bisquare');
coeff06 = coeffvalues(f06)
pvals = probvalues(f06)
disp(f06)
plot(f06,d2006,pf31.piezo_F31(r2006),'.b')  <--This is line 17 d2006 is a datenum value
xlabel('Date');
ylabel('Water Level (m)')
title('P31');
ylim([-10 -2]);

Answer 1

[Walter Roberson]

It is not possible to plot datetime (pf31.Date1) and datenum (d2006) on the same axis.

Try using datetime() isntead of datenum() for d2006

[Thomas Burbey]

It worked fine with revision R2016a.

[Walter Roberson]

R2016a converted the datetime to datenum for plot() purposes. In later versions it uses datetime based axis