[R] problem with axis alignment when plotting 2 time series on same graph

[Uwe Ligges]

Josef.Kardos at phila.gov wrote:
> I am trying to plot 2 time series on the same graph. 
> For example, X1 is the vector of dates and times, its class is POSIXt. Y1 
> is an environmental parameter, e.g. salinity. X2 is a second vector of 
> dates and times, also of class POSIXt.    X2 has a different length than 
> X1, but they have  the exact same range.  Y2 is another environmental 
> parameter, e.g. stream flow rate.
> 
> My approach is basically like this:
> plot(X1,Y1)
> par(xaxs = "r")
> plot(X1, Y1)
> par(new=TRUE)
> plot(X2, Y2)

You probably want to use lines() to add the second time series if the 
dates in X1 and X2 are not of identical range.

Uwe Ligges



> the result is that the date tick marks for the second graph do not exactly 
> align with the first graph. This is because the second time series is 
> being slightly 'squeezed'; its timescale should match the first graph but 
> for some reason R is plotting it on a smaller scale. 
> The strange thing is that this doesn't happen for all time series I plot. 
> For example X3 and Y3 which have the same basic characteristics as X2 and 
> Y2, overlay perfectly with X1 and Y1. I don't know why it works for some 
> but not others.
> 
> I also tried setting xaxs to "i" -- and had the same outcome. Also had the 
> same outcome when trying either  par(xaxs="i") or par(xaxs="r") after the 
> command par(new=TRUE)
> 
> Any insights would be appreciated
> 
> JK
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> 
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