function curve() swap axes

An embedded and charset-unspecified text was scrubbed...
Name: not available
URL: <https://stat.ethz.ch/pipermail/r-help/attachments/20120705/a04445cf/attachment.pl>
Dear all,

I'm using the curve() function to plot discharge Q against water depth a.
However, I would like to have a graph of water depth a plotted against
discharge Q. How can this be done?
curve() is designed for plotting y vs x where y is a function of x, so I 
think you need to do it yourself.  Simply compute a vector of "a" values 
using seq(), compute the corresponding Q values using your formula, and 
then use

plot(Q, a, type="l")

You can add the lwd and other arguments too if you like.

Duncan Murdoch
Minimal working example:
S0 = 0.004
n = 0.04
tanalpha = 1.4/1.5
par(mar = c(5,5,1,1)) # b, l, t, r
curve((sqrt(S0)/n)*(0.035+(0.7+(x-0.1)/tanalpha)*(x-0.1))*((0.035+(0.7+(x-0.1)/tanalpha)*(x-0.1))/(2*sqrt((0.7/2)^2+0.1^2)+2*sqrt((x-0.1)^2+((x-0.1)/tanalpha)^2)))^(2/3),0.1,1.55,
lwd = 3, col = "royalblue4", ann = F, axes = T)
title(xlab = parse(text='a~bgroup("[", m, "]")'))
title(ylab = parse(text='Q~bgroup("[", m^3/s, "]")'))
box()
I tried to find the inverse function, but that doesn't seem to exist.
Thank you for support,
Boudewijn

	[[alternative HTML version deleted]]

______________________________________________
R-help at r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.

Dear all,

I'm using the curve() function to plot discharge Q against water  
depth a.
However, I would like to have a graph of water depth a plotted against
discharge Q. How can this be done?
Minimal working example:
S0 = 0.004
n = 0.04
tanalpha = 1.4/1.5
par(mar = c(5,5,1,1)) # b, l, t, r
curve((sqrt(S0)/n)*(0.035+(0.7+(x-0.1)/ 
tanalpha)*(x-0.1))*((0.035+(0.7+(x-0.1)/tanalpha)*(x-0.1))/ 
(2*sqrt((0.7/2)^2+0.1^2)+2*sqrt((x-0.1)^2+((x-0.1)/ 
tanalpha)^2)))^(2/3),0.1,1.55,
lwd = 3, col = "royalblue4", ann = F, axes = T)
title(xlab = parse(text='a~bgroup("[", m, "]")'))
title(ylab = parse(text='Q~bgroup("[", m^3/s, "]")'))
box()
I tried to find the inverse function, but that doesn't seem to exist.
R does not perform computer algebra. If you wanted a numerical  
approach, you can construct a close fit to that function with  
approxfun() and then reverse the x and y roles to create an inverse.  
Actually, since curve returns a list with x and y components you could  
also do this:

xycurv <- curve((sqrt(S0)/n)*(0.035+(0.7+(x-0.1)/ 
tanalpha)*(x-0.1))*((0.035+(0.7+(x-0.1)/tanalpha)*(x-0.1))/ 
(2*sqrt((0.7/2)^2+0.1^2)+2*sqrt((x-0.1)^2+((x-0.1)/ 
tanalpha)^2)))^(2/3),0.1,1.55,
+ lwd = 3, col = "royalblue4", ann = F, axes = T)

plot(xycurv$y, xycurv$x)
David Winsemius, MD
West Hartford, CT
An embedded and charset-unspecified text was scrubbed...
Name: not available
URL: <https://stat.ethz.ch/pipermail/r-help/attachments/20120706/18646c32/attachment.pl>