def draw
  background(200)
  maxX = 2.8
  maxY = 1.5

  # Compute the natural log of a value between 0 and maxX
  xValue = map(mouseX, 0, width, 0, maxX)
  if xValue > 0
    # Cannot take the log of a negative number.
    yValue = log(xValue)
    y = map(yValue, -maxY, maxY, height, 0)

    # Display the calculation occurring.
    legend = format("log(%.2f)\n=%.3f", xValue, yValue)
    stroke(150)
    line(mouseX, y, mouseX, height)
    fill(0)
    text(legend, 5, 15)
    noStroke
    ellipse(mouseX, y, 7, 7)
  end

  # Draw the log(x) curve,
  # over the domain of x from 0 to maxX
  noFill
  stroke(0)
  shape do
    (0...width).each do |x|
      xValue = map(x, 0, width, 0, maxX)
      yValue = log(xValue)
      y = map(yValue, -maxY, maxY, height, 0)
      vertex(x, y)
    end
  end
  line(0, 0, 0, height)
  line(0, height / 2, width, height / 2)
end