# Stair step interpolation in R

## 2007-11-15

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

plot(0:5,5:0) lines(0:5,5:0,type="s")

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

plot(0:5,5:0) lines(0:5,5:0,type="S")

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

plot(0:5,5:0) lines(0:5,5:0,type="l")

to get this plot This plot shows a linear transition between (5,0) and (4,1).