[R] Lattice: Understanding How Points Connected by Lines

[Rich Shepard]

   A data frame has one factor, one date, and one numeric column. When I plot
these using the default pch of the open circle (first attachment),

xyplot(TDS ~ sampdate | she.s, data = sheep.cast, main = 'TDS in Sheep
Creek', ylab = 'Concentration (mg/L)', xlab = 'Time')

I see the higher concentration points toward the right on several panels.
But, when I change to using a line type (second attachment),

xyplot(TDS ~ sampdate | she.s, data = sheep.cast, type = 'l', main = 'TDS in
Sheep Creek', ylab = 'Concentration (mg/L)', xlab = 'Time')

the higher concentrations are connected to points far to the left (earlier
years). I expected to see a line with a few spikes toward the right sides of
the panels.

   Where do I start to look for the reason for this behavior?

TIA,

Rich
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