I plotted contour plot of this formula by Wolfram Alpha $$\log(\frac{a+1}{c+1}), 0 < a < 1000, 0 < c < 1000$$ the contour plot of the above formula
In the above figure, there is a crooked line in the bottom part. If I plot this, there is no crooked line.$$\log(\frac{a}{c}), 0 < a < 1000, 0 < c < 1000$$
Why there is crooked line in the first case?