search for: unequispaced

Dear R users, I want to plot a one variable continuous function f(x) vs x, x=[0,1]. Say for example: f(x)= x^2. Now, using the command plot(f~x) I will get a curve where the range of x-axis is [0,1] with all equispaced label. But, I need something else, and that is: my curve will be such that 80% on x-axis the range would be [0,0.5] and the rest 20% would contain [0.5,1]. Let me draw informally