Circos > Documentation > Tutorials > Quick Start > Ticks And Labels
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Use the latest version of Circos and read Circos best practices—these list recent important changes and identify sources of common problems.
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1 — Quick Start

2. Ticks & Labels

In this tutorial, I will add labels to the ideograms as well as ticks and tick labels.

To do this, the <ideogram> block needs to be expanded to include label parameters. Tick labels are defined in a different block, <ticks>.

ideogram labels

Labels for ideograms can be placed at any radial position and formatted flexibly. The minimum label definition is shown below.

<ideogram>

<spacing>
default = 0.005r
</spacing>

# Ideogram position, fill and outline

radius           = 0.90r
thickness        = 20p
fill             = yes
stroke_color     = dgrey
stroke_thickness = 2p

# Minimum definition for ideogram labels.

show_label       = yes
# see etc/fonts.conf for list of font names
label_font       = default 
label_radius     = 1r + 75p
label_size       = 30
label_parallel   = yes

</ideogram>

The radial position of the labels can be adjusted using label_radius.

# relative to outer ideogram radius 
label_radius = 1.1r
label_radius = 0.8r
# absolute position
label_radius = 500p
# relative position with absolute offset
label_radius = 1r + 100p
# using image dimensions
label_radius = dims(image,radius) - 50p
# using ideogram position
label_radius = dims(ideogram,radius_outer) + 50p
label_radius = dims(ideogram,radius_inner) - 50p

In other parts of Circos configuration, positional parameters (e.g. inner/outer radius of data tracks) support compound expressions that combine absolute and relative values (e.g. 1r+50p).

The quantity used as the reference for relative units depends on which parameter is defined. It is usually defined as the "parent container" of the element. For example, when definition ideogram position, the reference is image radius. When using track position, the reference is ideogram radius. As a result, when the parent element is moved (e.g. ideogram), all other elements move with it (e.g. data tracks).

tick marks and labels

Ticks are defined by group. You can have absolute or relatively spaced ticks, as well as ticks at specific positions. An entire tutorial is dedicated to ticks. Here, I show a minimum definition.

show_ticks          = yes
show_tick_labels    = yes

<ticks>

radius           = 1r
color            = black
thickness        = 2p

# the tick label is derived by multiplying the tick position
# by 'multiplier' and casting it in 'format':
#
# sprintf(format,position*multiplier)

multiplier       = 1e-6

# %d   - integer
# %f   - float
# %.1f - float with one decimal
# %.2f - float with two decimals
#
# for other formats, see http://perldoc.perl.org/functions/sprintf.html

format           = %d

<tick>
spacing        = 5u
size           = 10p
</tick>

<tick>
spacing        = 25u
size           = 15p
show_label     = yes
label_size     = 20p
label_offset   = 10p
format         = %d
</tick>

</ticks>

The primary parameter in each <tick> block is spacing. This defines the distance between adjacent ticks in this group. Typically, this value is defined in terms of chromosomes_units parameter — the suffix u is used for this — to keep the number legible.

If a tick belongs to multiple groups, the group with largest spacing is prefered. Thus, the tick at 50 Mb will take its formatting from the spacing=25u group, not the spacing=5u group.