A unit is a measurement. for example, a unit could be an inch, or it could be a foot. to convert you need the exact measurement so if a line is 12 inches it's 1 foot. if it is 13 then it stays in inches. You convert units to the easiest variable to calculate and then you solve your equation from there.
Hope I helped! :)
The equation of proportional relationship is of the form:

where m is the slope.
Drawing it is fairly easy, you start from the origin (0, 0) and you draw a diagonal staight line. The inclination of the line depends on the value of m. See picture attached.