Plotting lots of lines is not the correct solution, you should use the grid command, or set the axes grid properties (e.g. adjust tick-mark positions, grid linestyle, grid color, grid transparency, etc.). The MATLAB documentation gives examples of how to adjust the grid lines:
This should get you started:
>> x = -10:0.1:10;
>> y = 100*sin(x);
>> plot(x,y,'-.')
>> ylim([-150,150])
>> set(gca,'YGrid','on') % Turn on the Y-grid. Also: color, linestyle, etc.
Produces this plot:
