It is quite strange that mathwork has made it quite difficult to extract data out of plotResiduals function or plot. The following code shows one way. The following code shows one way to change the width of the bars by extracting the data from the plotResiduals figure and then creating a new figure using bar() function.
load carsmall
tbl = table(MPG,Weight);
tbl.Year = categorical(Model_Year);
mdl = fitlm(tbl,'MPG ~ Year + Weight^2');
p = plotResiduals(mdl);
vert = p.Vertices;
vert(1:5:end,:) = [];
hist_data = blockproc(vert, [4 2], @(x) mean(x.data(2:3,:)));
bar(hist_data(:,1), hist_data(:,2), 'BarWidth', 0.7);
