>> str{1}
ans =
cos(x)
Line 2
>> size(str{1})
ans =
2 6
Of course you should not be using strvcat anyway, as the documentation clearly states: "*Note:* strvcat is not recommended. Use char instead."
The real topic is that you apparently want multiple lines in the XTickLabels, something that is MATLAB version dependent. So if you actually read the help for your installed MATLAB version then you wold learn if newlines are supported in your MATLAB, and how. If you want us to help you, then please tell us in a comment: what version of MATLAB are you using?