30 Sep 2010 crhodes   » (Master)

My weekly diary schedule has already slipped! In my defence, last week was exceptional, because the major activity was neither entrepreneurial nor academic, but practical and logistical: moving house. A lengthy rant about the insanity of the English conveyancing system is probably not of great interest to readers of this diary, so I will save the accumulated feelings of helplessness and insecurity for some other outlet.

Meanwhile, back to work. It's the start of teaching next week; fortunately, I am teaching largely the same material as last year, so now is the time that I can reap the benefits of preparation time that spent on the course over the last two years. Inevitably, there will be new things to include and outdated material to remove or update, but by and large I should be able to deliver the same content.

This is a relief, because of course this year I only have one fifth of my time on academic-related activities. This means that various things have to be sacrificed or delegated, not least some of my extra-curricular activities such as being release manager of SBCL – so I'm very glad that Juho Snellman has volunteered to step in and do that for the next while. (He suffered the by-now traditional baptism of fire, dealing with amusing regressions late in the 1.0.42.x series, and released version 1.0.43 today; we'll see how his coefficient of grumpiness evolves over the next few months).

In the land of industry, what I've mostly been doing is drawing graphs. As the screenshot in last week's my previous entry suggests, I'm using R for data processing and visualisation; I have datasets with large numbers of variables, and the facilities for visualising those quickly and compactly with the lattice package (implementing Becker and Cleveland's trellis paradigm) are very convenient. By and large, progressing from prototype visualisation to presentation- or publication-quality visualisation is also straightforward, but I spent so long figuring out one thing that I needed to do this week that I'll document it here for posterity: that thing was to construct a graph using lattice with an axis break. It's not that it's absurdly difficult – there are plenty of hookable or parameterisable functions in the lattice graph-drawing implementation; the difficult part is finding out which functions to override, which hooks to use, and which traps to avoid.

The problem as I have found it is that when drawing a lattice plot, for these purposes, things happen in an inconvenient order. First, the axes, tickmarks and labels are drawn, using the axis function provided to the lattice call (or axis.default by default; then the data are plotted using the panel function. So, that would be fine; one could even hackily draw over the axis in the panel function to implement the axis break, at least if one remembers to turn clipping off with clip=list(panel="off") in par.settings. Except that the axis function doesn't actually draw the axis lines; instead, there's a non-overridable bit of plot.trellis which draws the box around the plot, effectively being the axis lines – and that happens after everything else.

So, piling hack upon hack: there's no way of not drawing the box. There is, however, a way of drawing the box with a line thickness of zero: pass axis.line=list(lwd=0) in par.settings as well. Ah, but then the tick marks have zero thickness too. Oh, but we can override that setting of axis.line$lwd within our custom axis function. (Each of these realisations took a certain amount of time, experimentation, and code reading to come to pass...). What it boils down to, in the end, is a call like

       screens=1, col=c(2,3,4), lwd=2, lty=3, more=TRUE,
       ylim=c(0.5,1.7), scales=list(
                          x=list(at=dates, labels=date.labels),
                            labels=c("- 50%", "± 0%", "+ 50%", "+ 100%"))),
         text=list(lab=c("5m", "500k", "galileo"))),
       par.settings = list(axis.line=list(lwd=0),
         clip=list(panel="off", strip="off")),
       axis=function(side, scales, components, ...) {
         lims <- current.panel.limits()
         panel.axis(side=side, outside=TRUE, at=scales$at,
                    draw.labels=side %in% c("bottom", "left"), rot=0)
         panel.lines(lims$xlim[[1]], lims$ylim, col=1, lwd=1)
         panel.lines(lims$xlim[[2]], lims$ylim, col=1, lwd=1)
         panel.lines(c(lims$xlim[[1]], as.Date("2010-09-11")+0.45),
                     lims$ylim[[1]], col=1, lwd=1)
         panel.lines(c(lims$xlim[[2]], as.Date("2010-09-11")+0.55),
                     lims$ylim[[1]], col=1, lwd=1)
         panel.lines(c(lims$xlim[[1]], as.Date("2010-09-11")+0.45),
                     lims$ylim[[2]], col=1, lwd=1)
         panel.lines(c(lims$xlim[[2]], as.Date("2010-09-11")+0.55),
                     lims$ylim[[2]], col=1, lwd=1)
       panel=function(x,y,...) {
         xs <- current.panel.limits()$xlim
         ys <- current.panel.limits()$ylim
                       border="white", col="white", alpha=1)
                     c(ys[1]-0.025,ys[1]+0.025), col="black")
                     c(ys[2]-0.025,ys[2]+0.025), col="black")
                     c(ys[2]-0.025,ys[2]+0.025), col="black")
                     c(ys[1]-0.025,ys[1]+0.025), col=1, lwd=1)
                    sprintf("%2.0f%%", round(100*t(gmeans.zoo[1,]-1))),

allows me to draw a picture like

for us to show to interested parties.

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