forcats fct_reorder2() in R: Order Factors for Legends

The forcats fct_reorder2() function reorders factor levels by the y values at the largest x, so a line chart legend lists groups in the same order the lines appear at the right edge of the plot.

aes(color = fct_reorder2(grp, x, y))      # match legend to line order
fct_reorder2(grp, x, y)                   # reorder by y at the last x
fct_reorder2(grp, x, y, .fun = first2)    # reorder by y at the first x
fct_reorder2(grp, x, y, .desc = FALSE)    # ascending instead of descending
fct_reorder2(grp, x, y, .fun = last2)     # default helper, stated explicitly
levels(fct_reorder2(grp, x, y))           # inspect the resulting order

What fct_reorder2() does in one sentence

fct_reorder2() orders factor levels for two-dimensional plots. You give it a factor and two numeric vectors, an x and a y. For each level it finds the y value sitting at the largest x value, then sorts the levels by those y values. The data stays untouched. Only the order of the levels changes.

This solves a specific annoyance. In a line chart where a factor sets the line color, ggplot2 builds the legend in factor level order, which is alphabetical by default. The lines themselves end up stacked by their y values on the right edge. When those two orders disagree, a reader's eye has to bounce between the legend and the lines to match them. fct_reorder2() aligns the legend with the lines so the chart reads itself.

Syntax

fct_reorder2() takes a factor plus an x and y pair. The signature is compact, and most calls only need the first three arguments.

The arguments work as follows.

• .f: the factor whose levels you want to reorder.
• .x: a numeric vector, usually the variable on the plot's x axis.
• .y: a numeric vector, usually the variable on the y axis.
• .fun: the summary helper. The default last2 reads y at the largest x. Pass first2 to read y at the smallest x.
• .desc: sort descending. The default TRUE puts the highest line at the top of the legend.

The helpers last2() and first2() exist only to serve fct_reorder2(). last2() returns the value of y when the pair is sorted by x and you take the last row. first2() takes the first row instead.

fct_reorder2() examples

Start with a line chart that has a mismatched legend. The built-in ChickWeight data records chick body weight over time across four diets. Summarising to a mean per diet per time point gives four clean lines.

Plot the lines colored by Diet and the legend lists diets 1, 2, 3, 4, even though diet 3 finishes highest. Wrapping Diet in fct_reorder2() fixes the order.

You can confirm the new order without drawing anything by calling levels() on the reordered factor.

To order by where the lines start rather than end, swap in the first2 helper. This is useful when the left edge of the plot is the focus.

fct_reorder2() vs fct_reorder()

The two functions cover different plot shapes. fct_reorder() is for one-dimensional displays where the factor maps to a position, such as the axis of a bar chart or boxplot. fct_reorder2() is for two-dimensional displays where the factor maps to a non-position aesthetic like color.

The decision rule is simple. If the factor sits on an axis, reach for fct_reorder(). If the factor sets a color or linetype on a chart with both x and y, reach for fct_reorder2().

Common pitfalls

Three mistakes account for most confused legends. Each has a quick fix.

Using fct_reorder() on a line chart passes only one variable, so it sorts by the median across all time points rather than the endpoint. The legend then disagrees with the lines.

Forgetting that .desc = TRUE is the default surprises people who expect ascending order. The highest line goes to the top of the legend on purpose. Set .desc = FALSE to flip it.

Passing x and y in the wrong order silently produces a valid but wrong ordering. The second argument is always x, the third is always y. fct_reorder2(Diet, Time, weight) reads "order Diet by weight at the largest Time."

Try it yourself

Related forcats functions

fct_reorder2() belongs to a family of ordering helpers. Pick the one that matches how your plot uses the factor.

• fct_reorder(): order levels by a summary of one variable.
• fct_infreq(): order levels from most to least frequent.
• fct_inorder(): order levels by first appearance in the data.
• fct_relevel(): move specific levels to chosen positions.
• fct_rev(): reverse the existing level order.

See the official forcats reference for the full argument list and the last2/first2 helper details.

FAQ

What is the difference between fct_reorder and fct_reorder2?

fct_reorder() reorders a factor using a single numeric variable and is meant for one-dimensional plots like bar charts and boxplots. fct_reorder2() reorders using two variables, an x and a y, and is meant for two-dimensional plots where the factor controls color. Use fct_reorder2() whenever you need a line chart legend to match the order of the lines.

What do last2 and first2 do in fct_reorder2?

They are summary helpers that pick a single y value per level. last2() sorts the x and y pair by x and returns the y at the largest x, which is the default. first2() returns the y at the smallest x instead. You choose between them with the .fun argument depending on whether the end or the start of the plot matters.

Why does my fct_reorder2 legend still not match the lines?

Check that you passed x and y in that exact order, since the second argument is x and the third is y. Also confirm you used fct_reorder2() and not fct_reorder(), because the single-variable version sorts by a median across all points rather than the endpoint. The mismatch usually traces back to one of those two slips.

Does fct_reorder2() change my underlying data?

No. fct_reorder2() only changes the order of the factor's levels, which is metadata attached to the factor. The values themselves, their length, and every other column stay exactly the same. Reordering levels is purely a display concern and is safe to do inside aes() at plot time.