[R] plotting 2 Y-axes, aligning the 0 value

[Frederik Vanrenterghem]

Hi,

Being a novice to R, I would like to create a graph in R with 2 axes.
One of the 2 only has positive values, the other one also has negative
values. The part I'm struggling with is how to align the 2.

Rather than starting to plot the data from the x axis, I would like to
start plotting the positive values on the right axis only as of the 0
value on the left axis.

Using a simple example found online, I understand how to get 2 axis on
a single drawing, but I'm unsure how to accomplish the alignment
objective. In the example, the 0 value of the right axis should align
with the 0 of the left axis, and the red line should start in the
middle of the graph only, rather than at the bottom of it.

(This is an arbitrary example - the right axis could also start as of
value 1000, in which case I'd like 1000 to align with 0 of the left
axis.)

Code:

# set up some fake test data
time <- seq(0,72,12)
betagal.abs <- c(0.05,-0.18,0.25,0.31,0.32,0.34,0.35)
cell.density <- c(0,1000,2000,3000,4000,5000,6000)

#add extra space to right margin of plot within frame
par(mar=c(5, 4, 4, 4) + 0.1)

# Plot first set of data and draw its axis
plot(time, betagal.abs, pch=16, axes=F, ylim=c(-1,1), xlab="",
ylab="", type="b",col="black", main="Mike's test data")
axis(2, ylim=c(-1,1),col="black")
mtext("Beta Gal Absorbance",side=2,line=2.5)
box()

# Allow a second plot on the same graph
par(new=T)

# Plot the second plot and put axis scale on right
plot(time, cell.density, pch=15,  xlab="", ylab="", ylim=c(0,7000),
axes=F, type="b", col="red")
mtext("Cell Density",side=4,col="red",line=2.5)
axis(4, ylim=c(0,7000), col="red",col.axis="red")

# Draw the time axis
axis(1,pretty(range(time),10))
mtext("Time (Hours)",side=1,col="black",line=2.5)

# Add Legend
legend(5,7000,legend=c("Beta Gal","Cell
Density"),text.col=c("black","red"),pch=c(16,15),col=c("black","red"))

Thanks in advance for any help you can provide!

Best regards,
Frederik Vanrenterghem