When plotting drug concentration results on a semilogarithmic graph, you use the same method to plot the points as you would use when plotting points on a graph using the more usual linear axes: you are still plotting CONCENTRATION against time.
To find the x-axis position, note the time and find its value on the x-axis.
To find the y-axis position, note the concentration and find its value on the y-axis. It's just that the position on the y-axis will seem a little unusal compared to when using a linear axis.