code: function integral(x0, x1, slope, intercept) {
                  return (0.5 * slope * x1 * x1 + intercept * x1) - (0.5 * slope * x0 * x0 + intercept * x0);
              }
docs: Function that calculates the integral of the line between two
  x-coordinates, given the slope and intercept of the line.
  @param x0
  @param x1
  @param slope
  @param intercept
  @returns {number} integral value.