[R] How to convert line plot to lattice xtplot?

[Luigi Marongiu]

Hello,
I have drawn data from a dataframe as follows:
```
Substance = rep(c("A", "B", "C", "D"),4)
Concentration = rep(1:4,4)
Value = c(62.80666667, 116.26333333,  92.26000000,   9.87333333,  14.82333333,
92.37333333, 98.95666667,   1.48333333,   0.64666667,  50.66000000,
25.75333333,   0.69000000, 0.21666667,   7.40666667,   6.92000000,
0.06333333)
df = data.frame(Substance, Concentration, Value, stringsAsFactors = FALSE)
Value = c(15.2974126, 16.3196089, 57.4294280,  9.1943370, 20.5567321,
14.0874424,
38.3626672, 0.3780653,  0.4738495, 37.9124874, 16.2473916,  0.7218726,
0.2498666,  8.4537585, 10.8058456,  0.1096966)
dfsd = data.frame(Substance, Concentration, Value, stringsAsFactors = FALSE)
COLS = c("gold", "green", "purple", "blue")

plot(df$Value[df$Substance == "A"] ~ unique(df$Concentration),
xlab=expression(bold(paste("Concentration (", mu, "M)"))),
ylab=expression(bold("Value")),
type = "o", col = "gold", pch = 16,
xaxt="n", las=3, log = "x",
ylim = c(0, max(df$Value+dfsd$Value))
)
points(df$Value[df$Substance == "B"] ~ unique(df$Concentration),
type = "o", col = "green", pch = 16)
points(df$Value[df$Substance == "C"] ~ unique(df$Concentration),
type = "o", col = "purple", pch = 16)
points(df$Value[df$Substance == "D"] ~ unique(df$Concentration),
type = "o", col = "blue", pch = 16)
legend("topright", legend = LETTERS[1:4],
lty = 1, pch = 16,
col = COLS)
```
I then tried to use lattice but I got a stripplot instead. At each x
(1:4) only one group (A:D) is drawn. What am I missing?
```
library(lattice)
xyplot(Value ~ Concentration,
group = Substance, data = df,
pch = 16, cex = 1.2, type = "b",
xlab=expression(bold(paste("Concentration (", mu, "M)"))),
ylab=expression(bold("Infection rate")),
col=COLS,
scales = list(x = list(log = 10, at=c(unique(df$Concentration))
)
),
key = list(space="top", columns=4, col = "black",
points=list(pch=c(16, 16, 16, 16),
col=COLS
),
text=list(c("A", "B", "C", "D")
)
)
)
```

Thank you

-- 
Best regards,
Luigi