Circos > Documentation > Tutorials > 2d Tracks > Scatter Plots

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7 — 2D Data Tracks

1. Scatter Plots

Syntax for 2D data types is similar to that for links. You define all 2D plots within >plots< block, with one plot per <plot> block. Within a plot block, you define formatting and data file for the plot, and can insert optional >rules> block that contains multiple <rule> blocks.

There are several 2D data types and the scatter plot is one. The image for this example contains three scatter plots, all derived from the same data file. Each plot is formatted slightly differently. The data are SNP density for windows of 100kb (number of SNPs from dbSNP per base).

default settings

Each plot type has default settings which you can find in etc/tracks. For example, 

# etc/tracks/scatter.conf
glyph            = circle
glyph_size       = 10
color            = grey
stroke_color     = black
stroke_thickness = 0
r1               = 0.79r
r0               = 0.7r
orientation      = out

These settings are applied to every <plot> block of type=scatter. YOu can undefine a setting using undef, 

stroke_color = undef

basic scatter plot syntax

2D data types share have the same format parameters, with a few exceptions. For the scatter plot, a basic definition might look like this 


<plots>

<plot>

show  = yes
type  = scatter

file  = data/6/snp.density.txt
r1    = 0.75r
r0    = 0.90r
max   = 1.0
min   = 0.0

glyph            = rectangle
glyph_size       = 8
color            = red
stroke_color     = dred
stroke_thickness = 1

</plot>

</plots>

The parameters are

  • show - as with highlights or links, determines whether to draw the plot or not
  • type - determines the type of plot and can be one of scatter, line, histogram, heatmap, etc.
  • file - file that contains plot data
  • min/max - the range of the plot axis, data outside this range are clipped
  • r0/r1 - the inner and outer radius of the plot track, which may be formatted in absolute or relative (or mix) form, just like for highlights
  • glyph - shape of glyph to use for the scatter plot and can be one of circle, rectangle, or triangle
  • glyph_size - size of the glyph, in pixels
  • color - if used, the the glyph will be solid and of this color
  • stroke_color - if used, the glyph will have an outline of this color
  • stroke_thickness - if used, the glyph's outline will have this thickness, in pixels

The data file format for 2D plots is 

chr start end value options

Note that a value is associated with an explicit span, and not a single base pair position. For example, 

# value 0.005 at span 1000-2000
hs1 1000 2000 0.005
# value 0.010 at span 2001-2001, e.g. a single base position
hs1 2001 2001 0.010

The reason why Circos uses start and end positions for the data file is to maintain consistency between data and link files. Glyphs are drawn at the middle of the span. Thus, in the above example, the glyphs would be drawn at positions 1,500 and 2,001.

Points whose values fall outside of the min/max range defined in the plot parameters are not drawn. You do not need to write rules to exclude these points.

plot background

The r0 and r1 parameters define the radial start and end positions of the plot track. You can optionally add background fill(s) to the track with a <backgrounds> block. using the following parameters 

<backgrounds>
<background>
color = vvlgrey
</background>
</backgrounds>

By defining several <background> blocks, you can stripe and layer the background fills. Use y0/y1 to define the radial extent of the fill. 

<backgrounds>
<background>
color     = vlgrey
# absolute fill range
y1        = 0.5
y0        = 0
</background>
<background>
color     = vlred
# relative fill range
y1        = 1r
y0        = 0.75r
</background>
</backgrounds>

plot axis

You can draw a set of y-axis grid lines within your plot track using the <axes> block. 

<axes>
<axis>
color     = grey
thickness = 1
spacing   = 0.05r
</axis>
</axes>

The axes are a series of concentric arcs with a given spacing. The spacing value can be absolute or relative.

plot format rules

Formatting rules work for plots in the same manner as for links. Within the condition field, use var(value) to represent the data value for a particular point.

For example, 

<rule>
condition    = var(value) > 0.5
stroke_color = dgreen
color        = green
glyph        = rectangle
glyph_size   = 6
</rule>

would turn all points whose value is larger than 0.5 to be rectangles of size 6 with a dark green outline and green fill.

You can hide certain data points by setting show=no within the rule.