 # Problems with axis break and ticks for dose response curves

Hello everyone,

I am using the drc package to calculate and plot dose response curves. I recently tried to plot curves with control data (i.e. x values of 0), but doing so gives me problems with the x-axis.

Here is some example:

library(drc)
library(data.table)

#x-axis data
dose <- c(0, 0, 0,
0.03, 0.03, 0.03,
0.08, 0.08, 0.08,
0.25, 0.25, 0.25,
0.75, 0.75, 0.75,
2, 2, 2,
6.67, 6.67, 6.67,
20, 20, 20)

#y-axis data
resp <- c(120,122,125,
99,98,95,
100,97,98,
99,97,98,
83,86,85,
46,41,49,
1,2,3,
0,0,1
)

#create data table with dose and response data
Data <- data.table(dose=dose, resp=resp)
#use drm function of drc package to get curve data (a drm object)
drm <- drm(resp ~ dose, data = Data, fct = llogistic2())

plot(drm,
lwd = 5, #curve thickness
col = "blue", #curve color
broken = TRUE, #use breakpoint in x-axis
bp = min(dose[dose!=0]), #breakpoint is supposed to start at second biggest dose
type = 'average', #draw average resp for each unique dose
ylim = c(0, 150), #range of y-axis
cex.main = 2, #title size
cex.lab = 2, #lable size
cex.axis =2 #axis lable size
)

If I run this code, I get the following plot:

I would like to solve 3 issues:

1. The x-axis tick after the axis break is not vertically aligned with the drawn response value (blue point above)
2. I would like to add logarithmic ticks between the existing ticks, but without adding any further numbers on the axis.
3. Is there a way to use the break and still have the curve starting from the control?

I already tried to achieve these by experimenting with drc.plot() and axis() parameters. Unfortunately, without any satisfying results. So it would be great, if someone can help me out!

Thank you,
Eike

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