I am working on some charts that show discrete (sudden) jumps at specific time points. This requires the use of stair step interpolation
- because if you just connected the lines
- it would imply a linear transition between consecutive points. Here’s an example of the plot I want.
It shows the number of patients at risk during any time point in the study. The number at risk has to be a whole number because it is impossible to have half a patient at risk in a study.
Here’s what it would look like if I just connected the dots.
The programming language R allows you to connect the points either a linear transition (which is what you want for most plots) or using a stair step transition. With a stair step transition
- you have the choice of moving first horizontally and then vertically
- or vice versa. If you specify a lower case s for the type of graph
- R will move horizontally first during the transition. If you specify an upper case S
- R will move vertically first during the transition. If you specify a lower case l
- R will use a linear transition between data points.
Here’s a simple example. The following R code
will produce this plot.
Notice that when you move from (5,0) to (4,1)
- you produce horizontally along y=5 and then drop down to 4 when x reaches 1. Change the R code to this
and you get this plot instead.
Now when you move from (5,0) to (4,1)
- you drop veritcally to 4 first and then proceed horizontally until you reach x=1. It a subtle difference
- but it is important. Finally
- use this code
to get this plot
This plot shows a linear transition between (5,0) and (4,1).