When a U.S. forecast says "70°F," it may be hard to tell how warm that is. Celsius and Fahrenheit are not directly proportional; they are connected by the linear relationship F = 1.8×C + 32. Move the slider to explore it.
The horizontal axis is Celsius (x) and the vertical axis is Fahrenheit (y). The point moves along the line y = 1.8x + 32, which has slope 1.8 and y-intercept 32. Move the slider and the point follows.
Why the formula looks this way: Water freezes at 0° Celsius but 32° Fahrenheit, and boils at 100° Celsius and 212° Fahrenheit. The line through (0,32) and (100,212) has the equation F=1.8C+32 — exactly a linear-function graph.