Welcome to the convoluted world of annotation panes.
You need to set the VerticalAlignment property to Bottom.
figure
plot(1:10)
dim = [.2 .5 .3 .3];
str = 'Straight Line Plot from 1 to 10';
h = annotation('textbox',dim,...
'String',str,...
'FitBoxToText','on',...
'VerticalAlignment','bottom');
Explanation
TL;DL The default text position within the annotation box starts in the upper-left of the textbox before the fit-box is applied. You have to indicate that the text should be in the lower-left of the box.
The default VerticalAlignment is "top" so let's take a look at what's going on when VerticalAlignment is Top and when the box is not fit to the text.
figure
plot(1:10)
dim = [.2 .5 .3 .3];
str = 'Straight Line Plot from 1 to 10';
h = annotation('textbox',dim,'String',str);
% Add axes that fills figure
ax2 = axes('Units','Normalized',...
'Position',[0 0 1 1],...
'visible','off', ...
'XLim', [0,1], ...
'YLim', [0,1]);
% Show the desired x and y position of the textbox
xline(ax2, dim(1))
yline(ax2,dim(2))
% Show desired textbox position and size
rectangle(ax2,'position', dim, 'linewidth', 2,'EdgeColor','r', 'LineStyle','--')
You can see why the text seems to be vertically displaced. The function applies the box-fit after the box is positioned.
Now let's set VerticalAlignment to Bottom,
figure
plot(1:10)
dim = [.2 .5 .3 .3];
str = 'Straight Line Plot from 1 to 10';
h = annotation('textbox',dim,'String',str,'VerticalAlignment', 'bottom');
% Add axes that fills figure
ax2 = axes('Units','Normalized',...
'Position',[0 0 1 1],...
'visible','off', ...
'XLim', [0,1], ...
'YLim', [0,1]);
% Show the desired x and y position of the textbox
xline(ax2, dim(1))
yline(ax2,dim(2))
% Show desired textbox position and size
rectangle(ax2,'position', dim, 'linewidth', 2,'EdgeColor','r', 'LineStyle','--')
Now when the fit-box is applied, the text is correctly positioned at the specified coordinate.
