The value of log a is the same as the value of the y intercept. To obtain this, we look on the graph for the point where the horizontal variable is 0. Remember that since the horizontal axis is logarithmic, the horizontal variable is actually log x, not just x, so we want the point where log x = 0. In order for log x to be 0, x must be 1. The y intercept can be read off the graph along the vertical line where x = 1.