Special Topics: Label Bars

For the gsn_csm graphical interfaces, setting cnFillOn = True, automatically creates a color bar.

lbLabelBarOn = False, Will turn off the label bar.

lbOrientation = "vertical", Moves the label bar to the right side vertical.

You can specify your own labels by setting lbLabelStrings equal to an array of strings. Here we chose other numbers, but the string could have any value.

lbLabelAngleF, Controls the angle of the labels while lbLabelFontHeightF, Controls the size of the labels.

A title can be added to the label bar by setting lbTitleOn equal to True, and providing a string using lbTitleString. The default position of the title is on top of the label bar. Example 4 demonstrates how to move the label bar title. The size of the label bar title can be changed with lbTitleFontHeightF.

In this example, we needed to move the entire label bar down b/c the oversized labels ran into the plot. pmLabelBarOrthogonalPosF adjust the label bar position perpendicular to the main plot axis. Note that this is a pm or plot manager resource.

lbTitlePosition = "Right", Moves the title from its default top position to the right. Other options include bottom and left. If the title position is top or bottom, the default orientation of the title string is across. It is down if the title position is left or right. Here we override this default by changing the title orientation to across using lbTitleDirection.

The default position of the labels for the gsn_csm interfaces is lbLabelAlignment = "InteriorEdges". We have changed this to the center using lbLabelAlignment = "BoxCenters".

The default label position is bottom, but this can be changed using lbLabelPosition.

The labels were made bolder by changing the font using lbLabelFont.

As we indicated in the first example, the gsn_csm graphical interfaces automatically create a label bar if color is turned on. If you use the generic graphical interfaces then you may need to create a label bar from scratch.

To turn on and modify a label bar, you must first set lbAutoManage = False, and pmLabelBarDisplayMode = "Always".

The default orientation of the label bar in NCL is Vertical, and the default side is Right. We change these with lbOrientation and pmLabelBarSide respectively.

The default label bar width and height is shorter and taller than our aesthetic preference. Additionally, the default label font height is huge, and a box is draw around the label bar. We have modified these features using pmLabelBarWidthF, pmLabelBarHeightF, lbLabelFontHeightF, and lbPerimOn.

The number of labels on the label bar can be adjusted by setting lbLabelStride to some integer value. You can also use lbLabelAutoStride which automatically determines if there is a label overlap and adjusts the stride accordingly.

The three resources (available since NCL version 4.2.0.a020) needed to reverse a colormap are: gsnSpreadColors= True
gsnSpreadColorStart= -1, starts the colormap at the last color. gsnSpreadColorEnd = 2, Begins the colormap at the beginning of the colormap.

sprintf is the function we can use to reformat the labels. Unfortunately, this has to be done after the plot is drawn because there is no way to do it a priori.

lbLabelAutoStride will automatically determine the best stride for a label bar if it senses an overlap.

gsn_labelbar_ndc is the function that will draw your own label bar. You can use any of the resources demonstrated on this page to adjust the style of the label bar.

The first frame draws solid-filled labelbars. The second frame draws pattern-filled labelbars by setting lbMonoFillPattern to False and setting lbFillPatterns to an array of fill patterns. The width of the pattern lines are increased by setting lbFillLineThicknessF = 2.0. The third frame increases the density of the fill pattern with lbFillScaleF = 0.5.

Note that gsnSpreadColors does not work on this label bar. See script for discussion and solution.

The old method (before V5.0.0) was to set cnLabelBarEndLabelsOn to True. This has been deprecated, and you should now set cnLabelBarEndStyle to "IncludeMinMaxLabels". This will turn on labels at the ends of a label bar, using the min and max values of your data.

gsn_create_labelbar is similar to gsn_labelbar_ndc except it returns an id that allows you to attach the labelbar to the plot using gsn_add_annotation.

The second frame shows how if you resize the plot, the labelbar automatically gets resized. Note that these two functions are only available in version 4.2.0.a034

A new contour resource called cnLabelBarEndStyle was added in V5.0.0. You can set this resource to one of three values to control how the end labelbar boxes are handled:

• "IncludeOuterBoxes" (frame 1, the default) - the first and last labels, representing the minimum and maximum contour levels, are aligned with the interior boundaries of the outer boxes.
• "IncludeMinMaxLabels" (frame 2) - the outer edges of the labelbar boxes are labeled with the min/max of the data.
• "ExcludeOuterBoxes" (frame 3) - the two outermost boxes are removed.

This detailed discussion is from a posting from Dave Brown, who also provided the script:

By default, with lbAutoManage set to True, the LabelBar code makes sure that all elements of the LabelBar fit inside the width and height specified using vpWidthF and vpHeightF. This requires that the LabelBar controls the font height of the labels. However, depending on other constraints, this often leads to text that is too small. To achieve control of the font height, you need to turn lbAutoManage off. Once the user controls the font height, the LabelBar can no longer ensure that everything will fit inside the specified width and height, but it still tries to if it can.

A significant resource is lbBoxMinorExtentF. This specifies the proportion of the space in the direction perpendicular to the labelbar orientation that is occupied by the labelbar. By default it is 0.33, or 1/3 of the width specified by vpWidthF in the case of a vertically oriented labelbar.

The total width is accounted for by five elements:

1. the left margin
2. the labelbar (box) width
3. the label offset (which specifies a space between the labelbar and the labels)
4. the width of the longest text string
5. the right margin

If all of these together amount to less than the width specified by vpWidthF, then the labelbar and the text are centered within the available width. This is what leads to the variable distance from the specified position of the top left corner of the labelbar.

To ensure that the top-left corner of the labelbar falls on the specified point, set the resources as follows:

lbJustification = "topleft"
lbLeftMarginF = 0.0

lbTopMarginF = 0.0 ; eliminate any margin at
                   ; the top and left of the labelbar

lbBoxMinorExtentF = 1.0 ; this makes the labelbar 
                        ; width equal to the full amount of vpWidthF.

As a side effect, this results in the labels being drawn entirely outside the labelbar viewport vpWidthF = .03 ; reduced from 0.1 to maintain approximately the labelbar width of the original plot. lbLabelOffsetF = .5 ; this specifies the offset between the labelbar and the labels as a fraction of lbLabelOffsetF (for a vertically oriented labelbar). Since lbLabelOffsetF has become smaller, this value needs to be increased to maintain approximately the original distance between the labelbar and the labels.