What is a function to represent a diagonal sine wave? I need to be able to plot pixels in this pattern.
To me, it looks like a sine wave pattern that is both diagonal and convergent.  



What would a function for that look like?  Thanks.
How about $x+\exp(-a x)\sin(b x)$ for suitable values of $a$ and $b$?

Here is a sample plot for $a=0.2$ and $b=10$.
It looks like you want almost a staircase type of line, so use this.

f(n)=sin(x)+x

It won't let me post unless I have more characters, so I'll throw in 

f(n)=sin(wx)a+dx where w is wavelength, a is amplitutde, and d is degree.