SVG line in y=mx+c format

The <line> tag only supports start and end attributes (x1, y1, x2, y2). As such, you'll need to pick x coordinates outside your canvas manually and use y2 = y1 + m(x2 - x1).

Edit

Looking through the spec, it's possible to transform individual elements as required:

<line ... transform="translate(x, y) rotate(theta)" />

Where theta is the angle of clockwise rotation in degrees.

So you could draw a long horizontal line from (-10000, 0) to (10000, 0), say, and then apply the appropriate rotation and translation to position it:

<line x1="-10000" y1="0" x2="10000" y2="0" transform="translate(150, 200) rotate(-30)" />

Will draw a (seemingly endless) line through (150, 200) of slope π/6 radians.