I'm trying to plot an exponential decay line (with error bars) onto a scatterplot in ggplot of price information over time. I currently have this:
f2 <- ggplot(data, aes(x=date, y=cost) ) + geom_point(aes(y = cost), colour="red", size=2) + geom_smooth(se=T, method="lm", formula=y~x) + # geom_smooth(se=T) + theme_bw() + xlab("Time") + scale_y_log10("Price over time") + opts(title="The Falling Price over time") print(f2)
The key line is in the geom_smooth command, of
formula=y~x Although this looks like a linear model, ggplot seems to automatically detect my scale_y_log10 and log it.
Now, my issue here is that date is a date data type. I think I need to convert it to seconds since t=0 to be able to apply an exponential decay model of the form
y = Ae^-(bx).
I believe this because when I tried things like y = exp(x), I get a message that I think(?) is telling me I can't take exponents of dates. It reads:
Error in lm.wfit(x, y, w, offset = offset, singular.ok = singular.ok, :
NA/NaN/Inf in foreign function call (arg 1)
log(y) = x works correctly. (y is a numeric data type, x is a date.)
Is there a convenient way to fit exponential growth/decay time series models within ggplot plots in the geom_smooth(formula=formula) function call?
geom_smooth(method="glm",family=gaussian(link="log"))- Ben Bolker 2012-04-03 20:33
Error in eval(expr, envir, enclos) : cannot find valid starting values: please specify someMittenchops 2012-04-03 20:42
This appears to work, although I don't know how finicky it will be with real/messy data:
set.seed(101) dat <- data.frame(d=seq.Date(as.Date("2010-01-01"), as.Date("2010-12-31"),by="1 day"), y=rnorm(365,mean=exp(5-(1:365)/100),sd=5)) library(ggplot2) g1 <- ggplot(dat,aes(x=d,y=y))+geom_point()+expand_limits(y=0) g1+geom_smooth(method="glm",family=gaussian(link="log"), start=c(5,0))