How can I change the thickness of tick marks in BarLegend?

Question

Bug introduced in 10.1 or earlier and persisting through 11.0.1 or later

---

This answer explains how to change various undocumented options for BarLegend with Method. In particular, I want to change the style of the ticks (marks and labels) in BarLegend. For example,

BarLegend[{"SunsetColors", {0, 1}}, LabelStyle -> {FontSize -> 12},
 Method -> {Frame -> False, TicksStyle -> Directive[Red, AbsoluteThickness[2]]}]

However, in Mathematica 10.2 on Linux, BarLegend refuses to change the thickness of the tick marks. Also, if I do not add the LabelStyle option then the labels don't change color? In Plot this command works,

Plot[Sin[x], {x, 0, 2 Pi},TicksStyle -> Directive[Red, AbsoluteThickness[2]]]

so I would expect that it works for BarLegend as well.

What is going on and is there a workaround to change the thickness of the tick marks? Thanks.

Answer 1

Somehow the AbsolutThickness you specified gets replaced by a default value of AbsoluteThickness[0.2].

This misbehavior can be corrected by replacing the incorrect value with your specification.

PlotLegends; (*preload definitions*)

Cell[BoxData[
   MakeBoxes@
      BarLegend[{"SunsetColors", {0, 1}}, LabelStyle -> {FontSize -> 12}, 
       Method -> {Frame -> False, TicksStyle -> Directive[Red, AbsoluteThickness[2]]}] /. 
     Directive[RGBColor[1, 0, 0], AbsoluteThickness[_]] -> 
      Directive[RGBColor[1, 0, 0], AbsoluteThickness[2]] // #[[1, 1]] &
   ], "Output"] // CellPrint

For opaque ticks:

Cell[BoxData[
   MakeBoxes@
      BarLegend[{"SunsetColors", {0, 1}}, LabelStyle -> {FontSize -> 12}, 
       Method -> {Frame -> False, TicksStyle -> Directive[Red, AbsoluteThickness[2]]}] /. 
     Directive[RGBColor[1, 0, 0], AbsoluteThickness[_]] -> 
      Directive[RGBColor[1, 0, 0], AbsoluteThickness[2], Opacity[1]] // #[[1, 1]] &
   ], "Output"] // CellPrint

---

Correcting the BarLegend of a DensityPlot, using the syntax provided in the answer by Praan :

DensityPlot[Sin[x y], {x, 0, 1}, {y, 0, 1}, 
  PlotLegends -> 
   BarLegend[Automatic, LabelStyle -> {FontSize -> 12}, 
    Method -> {Frame -> False, TicksStyle -> Directive[Red, AbsoluteThickness[2]]}]] /. 
 Placed[barLegend_BarLegend, args__] :> 
  Placed[ToExpression[
    FrameBox @@ MakeBoxes[barLegend] /. 
      Directive[Red, AbsoluteThickness[_]] -> 
       Directive[Red, AbsoluteThickness[2], Opacity[1]]], args]

The same output can be achieved by using the following LegendFunction

DensityPlot[Sin[x y], {x, 0, 1}, {y, 0, 1}, 
 PlotLegends -> 
  BarLegend[Automatic, LabelStyle -> {FontSize -> 12}, 
   Method -> {Frame -> False, 
     TicksStyle -> Directive[Red, AbsoluteThickness[2]]}, 
   LegendFunction -> (# /. 
       Directive[Red, AbsoluteThickness[_]] -> 
        Directive[Red, AbsoluteThickness[2], Opacity[1]] &)]]

---

With the answer by Praan and our discussion in the comments it became clear, that a wrong InterpretationFunction inside the TemplateBox created by BarLegend can cause additional problems.
Compare

MakeBoxes[
    BarLegend[{"SunsetColors", {0, 1}}, LegendMarkerSize -> 300, 
     LabelStyle -> {FontSize -> 12}, 
     Method -> {FrameStyle -> Black, AxesStyle -> None, 
       TicksStyle -> Black}]] /. 
   AbsoluteThickness[_] -> 
    AbsoluteThickness[2] /. (InterpretationFunction :> 
     f_) -> (InterpretationFunction :> (# &)) // ToExpression

with

MakeBoxes[
   BarLegend[{"SunsetColors", {0, 1}}, LegendMarkerSize -> 300, 
    LabelStyle -> {FontSize -> 12}, 
    Method -> {FrameStyle -> Black, AxesStyle -> None, 
      TicksStyle -> Black}]] /. 
  AbsoluteThickness[_] -> AbsoluteThickness[2] // ToExpression

or just the InterpretationFunction

MakeBoxes[
    BarLegend[{"SunsetColors", {0, 1}}, LegendMarkerSize -> 300, 
     LabelStyle -> {FontSize -> 12}, 
     Method -> {FrameStyle -> Black, AxesStyle -> None, 
       TicksStyle -> Directive[Black, AbsoluteThickness[2]]}]] /. 
   AbsoluteThickness[_] -> 
    AbsoluteThickness[2] // #[[-1, 2, 1]] & // ToExpression

and the first code block in the answer by Praan.

Answer 2

Update: You can also use the forms given in Options[Charting`iBarLegend] listed below, i.e., we can use Charting`TicksStyle for "TicksStyle", Charting`Frame for "Frame", etc.

BarLegend[{"SunsetColors", {0, 1}}, LabelStyle -> {FontSize -> 12}, 
 Charting`TicksStyle -> Directive[Opacity[1], Red, FontColor -> Black, AbsoluteThickness[5]],
 Charting`Frame -> False,
 Charting`TickLengths -> {20, 10}]

Note: This works for versions 9 thru 11 (Windows 10).

Original answer:

Use the (undocumented, afaik) options "TicksStyle" and "FrameStyle":

BarLegend[{"SunsetColors", {0, 1}}, LabelStyle -> {FontSize -> 12}, 
 "TicksStyle" -> Directive[Red, AbsoluteThickness[5]], 
 "FrameStyle" -> Opacity[0]]

In version 10, we need to add Opacity[1] in Directive[...]:

BarLegend[{"SunsetColors", {0, 1}}, LabelStyle -> {FontSize -> 12}, 
 "TicksStyle" -> Directive[Opacity[1], Red, FontColor->Black, AbsoluteThickness[5]], 
 "Frame" -> False]

A few more such options:

BarLegend[{"SunsetColors", {0, 1}}, 
 "LabelStyle" -> Directive[Green, Bold, FontSize -> Scaled[.4]],
  "Ticks" -> Range[0, 1, .1],
 "TicksStyle" -> Directive[Red, AbsoluteThickness[5]], 
 "FrameStyle" -> Directive[Dashed, Green], "TickSide" -> Left, 
 "LabelSide" -> Right, "Frame" -> True, "ImageSize" -> {100, 300}, 
 LegendMarkerSize -> {100, 500}]

Update: Full list of options

Options[Charting`iBarLegend]

Version 9:

Version 10:

Answer 3

Analysis

(Observations in 10.1.0 under Windows.)

Curiously it seems that AbsoluteThickness[0.2`] is hard-coded within the internal definitions. Formatting of BarLegend calls Charting`iBarLegend which calls Legending`LegendDump`iColorGradientLegend or Legending`LegendDump`iColorBandLegend. In the definition of iColorGradientLegend we find:

ticksstyle = Switch[ticksstyle,
  Automatic, {Directive[AbsoluteThickness[0.2`], GrayLevel[0.4`]], 
   Directive[AbsoluteThickness[0.2`], GrayLevel[0.4`], Opacity[0.3`]]},
  _?Charting`generalDirective | {_?
      Charting`generalDirective ..} | {}, (Charting`ConstructDirective[#1, 
      AbsoluteThickness[0.2`]] &) /@ Charting`padList[ticksstyle, 2],
  None | False, None,
  _, (Directive[AbsoluteThickness[0.2`], #1] &) /@ {GrayLevel[0.4`], 
    Directive[GrayLevel[0.4`], Opacity[0.5`]]}]

If the option value passed to this inner function is a "generalDirective" we specifically get:

(Charting`ConstructDirective[#1, 
  AbsoluteThickness[0.2`]] &) /@ Charting`padList[ticksstyle, 2]

So the program is appending AbsoluteThickness[0.2`] to anything we specify by way of Charting`ConstructDirective. This seems at best a poor design in my opinion.

Work-around

Seeing the cause of the problem inspires an unusual work-around; here is a chomp head that consumes the argument to its right and then vanishes:

chomp /: h_[a___, chomp[x__], b_, c___] := h[a, x, c]

Now when used on our AbsoluteThickness spec making sure it is the last:

BarLegend[{"SunsetColors", {0, 1}}, LabelStyle -> {FontSize -> 12}, 
 Method -> {Frame -> False, 
   TicksStyle -> Directive[Red, Opacity[1], chomp @ AbsoluteThickness[4]]}]

One could also enter Directive[chomp[ Red, Opacity[1], AbsoluteThickness[4] ] ].

Answer 4

I went back to the old legend functions that I used before BarLegend existed, and tried the following:

• First, copy the definitions of trimPoint, colorLegend, display and at by selecting the large code block in the section Color bar legend.

Then do this:

display[{
  colorLegend[
    ColorData["SunsetColors"],
    {-.5, .5},
    LabelStyle -> 
     Directive[Yellow, FontSize -> 12, FontWeight -> Bold],
    "ColorBarFrameStyle" -> Directive[AbsoluteThickness[2], Cyan],
    Background -> Darker@Darker@Darker@Blue,
    "ColorSwathes" -> None,
    Contours -> 10,
    RoundingRadius -> 15,
    BoxFrame -> 3,
    "Digits" -> 2] // at[{0, 0}, {1, 1}]},
 ImageSize -> 80,
 AspectRatio -> 4]

The thickness information can be put either into the option "ColorBarFrameStyle" or LabelStyle. To show how these options differ, I highlighted their targets in different colors (Yellow for LabelStyle and Cyan for the "ColorBarFrameStyle".

So this older legend functionality has the ability to customize thicknesses as you were asking. It doesn't require BarLegends, which makes it useable in Mathematica version 8, too. The workings of the positioning commands display and at are explained with more examples in the linked answer. Normally, you would use display to combine the legend bar with a figure. Then the AspectRatio and ImageSize options will have to be adjusted to fit both objects.

Answer 5

In the answer of Karsten 7. the BarLegend is shown as a Cell expression, and since I want to use the legend in an actual plot, it is not immediately useful. However, with the help of his/her answer, I managed to solve my problem.

First I make the legend:

barLegend = ToExpression[FrameBox@@MakeBoxes[
  BarLegend[{"SunsetColors", {0, 1}}, LegendMarkerSize -> 300,
    LabelStyle -> {FontSize -> 12},
    Method -> {FrameStyle -> Black, AxesStyle -> None, TicksStyle -> Black}]] /. 
AbsoluteThickness[_] -> AbsoluteThickness[2]];

I don't know how changing the head to FrameBox works, but without this, the thickness of the tick marks reverts back to the default value AbsoluteThickness[0.2] during ToExpression.

This legend can then be used in a plotting environment, for example DensityPlot:

DensityPlot[Sin[x+y^2],{x, 0, 3},{y, 0, 3},
  PlotLegends -> barLegend, LabelStyle -> {FontSize -> 12},
  ColorFunction -> ColorData["SunsetColors"], ImageSize -> 300]

Edit

See the update of Karsten 7's answer for a nice implementation; doing the replacement after the plot command has been evaluated.