there are a couple of ways to do this...
the 'dodgy' way
draw to a seperate DC and StretchBlt() to the screen.
The problem being that StretchBlt() is slow,
not good with line drawings (can lose some lines)
or non integer resizes (ie OK with 1:2 or 2:1 but bad with 1:3.454)
the 'correct' way
Look at MSDNs 'Coordinate Spaces and Transformations'
the 'quick' way
when drawing the axis find a ratio between the number of pixels and max range for that axis. Use this to convert each point, add to an array and draw with PolyLine() or similar.
ie
screen width = 640 pixels
X axis range (max X - min X) = 300
ratio = 2 ( 640/300 )
margin = 20 ( (640%300)/2) )
Don't forget to clear the area first.
