It depends on which time you want to plot.
First of all, you can put in x-axis the elapsed time in seconds: this can be useful for short tasks (no more that a few minutes) an is quite easy to read.
If your process is longer or if you want to have actual time on the axis, you may want to use string labels for the x-axis. This method is used for example in excel: x-axis value is given in fractions of a day, pesented in time format so that, for example, 0.25 gives 6 o'clock in the morning, 0.5 gives noon and so on.
As a priciple, you can use whichever unit you want for the x-axis and create your own set of labels to associate to some x values to display on the axis.
As an example, you can look at
this thread.
