As I understand it, you want to plot every 10th element of possibly different sized arrays. There are a few ways you could do this. One way would be to write a short function to filter your arrays for you, for instance:
plot_10 = @(x,y) plot(x(1:10:end),y(1:10:end));
plot_10(x1,y1);
plot_10(x2,y2);
...
EDIT: Just an additional thought. If you wanted to enable the extended functionality of plot (e.g. passing line/colour arguments, etc). You could do something like this:
plot_10 = @(x,y,varargin) plot(x(1:10:end),y(1:10:end),varargin{:});
plot_10(x1,t1,'k+');