[R] Overlaying data on graphs

Uwe Ligges ligges at statistik.uni-dortmund.de
Tue Oct 19 12:03:34 CEST 2004

par("usr") is your friend - both to ask for the height and also for 
setting a new height.

Uwe Ligges

Steven Murdoch wrote:

> [sorry, my last message seems to have been corrupted - here it is
> again.]
> I currently have a graph from network measurements of latency against
> time and would like to overlay some extra data on top, but with a
> different y scale, but I am not sure how to do this in an automated
> manner.
> The base graph is generated by these commands:
>  time=probe$rsec+(probe$rusec/1000000)
>  diff=((probe$rsec-probe$ssec)*1000000)+(probe$rusec-probe$susec)
>  plot(time[100:len],diff[100:len])
> Then I would like to overlay data on the usage of the network. In this
> data the x scale is the same (time), but the y axis is unrelated
> (buffer size). The approach I have currently been using is simply
> drawing on top of the graph:
>  lines(usage$time,(usage$bytes)*m+c,type="h",col="blue")
> The problem with this is that to make the lines visible, but not
> taking up the whole height, I need to manually tweak the m and c
> factors, based on the relative values of the usage and probe data.
> Since I have a lot of graphs to generate, I would like to do this
> automatically.
> I haven't been able to do this, because I can't find out the ymin and
> ymax of the graph generated by plot. If I could then it would be
> trivial to scale the factors so they took up say, 10% of the vertical
> space. There is par("yaxp"), but this shows the positions of the
> tickmarks, but these can be different from the actual ymin and ymax.
> Is there a way to do this?
> The alternative would be to have two graphs, one on top of the other.
> With the latency graph on top, taking up most of the space, and the
> usage graph below, with the scale of the n axis being identical.
> However I can't see how do this.
> Does anyone have any advice?
> Thanks in advance,
> Steven Murdoch.
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