3.2.8. Plot axis units and scales¶
Different units can be chosen for the quantities that are plotted. Which units are available depends upon the plot type that is used, for example an observed spectrum (data), a Differential Emission Measure distribution (DEM) etc. For the available plot types see Section Plot types.
Note also that depending upon the plot type, there are two or three axes available, the xaxis, yaxis and zaxis. The last axis is only used for contour maps or images, and corresponds to the quantity that is being plotted in the contours or image.
3.2.8.1. Xaxis units¶
3.2.8.1.1. Xaxis units for plot type model, data, chi, area, fwhm and spec¶
bin 
Bin number 
kev 
keV (kiloelectronvolt), default value 
ev 
eV (electronvolt) 
ryd 
Rydberg units 
hz 
Hertz 
ang 
Å (Ångstrom) 
nm 
nm (nanometer) 
vel 
Note: when using the velocity scale, negative values correspond to a blueshift, positive values to a redshift. Also note that in this case one needs to provide the reference energy or wavelength.
3.2.8.1.2. Xaxis units for plot type dem¶
bin 
Bin number 
kev 
keV (kiloelectronvolt), default value 
ev 
eV (electronvolt) 
ryd 
Rydberg units 
k 
K (Kelvin) 
mk 
MK (MegaKelvin) 
3.2.8.2. Yaxis units¶
3.2.8.2.1. Yaxis units for plot type model, spec¶
Photon numbers: 

cou 

kev 
(default value) 
ev 

ryd 

hz 

ang 

nm 

Energy units: 

wkev 

wev 

wryd 

whz 

wang 

wnm 

jans 
Jansky () 
units: 

iw 

ij 
Jansky Hz () 
Various: 

bin 
Bin number 
3.2.8.2.2. Yaxis units for plot type data¶
bin 
Bin number 
cou 

cs 

kev 
(default value) 
ev 

ryd 

hz 

ang 

nm 

fkev 

fev 

fryd 

fhz 

fang 

fnm 
3.2.8.2.3. Yaxis units for plot type chi¶
bin 
Bin number 
cou 

cs 

kev 
(default value) 
ev 

ryd 

hz 

ang 

nm 

fkev 

fev 

fryd 

fhz 

fang 

fnm 

dchi 
(Observed  Model) / Error (default) 
rel 
(Observed  Model) / Model 
3.2.8.2.4. Yaxis units for plot type area¶
bin 
Bin number 
m2 
(default) 
cm2 
3.2.8.2.5. Yaxis units for plot type fwhm¶
bin 
Bin number 
kev 
keV (kiloelectronvolt), default value 
ev 
eV (electronvolt) 
ryd 
Rydberg units 
hz 
Hertz 
ang 
Å (Ångstrom) 
nm 
nm (nanometer) 
de/e 
3.2.8.2.6. Yaxis units for plot type dem¶
The emission measure is defined as and is expressed in units of . Here is the electron density, is the Hydrogen density and the emitting volume.
bin 
Bin number 
em 
(default) 
demk 
, per keV 
demd 
, per K 
3.2.8.3. Plot axis scales¶
The axes in a plot can be plot in different ways, either linear or logarithmic.
For the yaxis we also have made an option for a mixed linear and logarithmic scale. The lower part of the plot will be on a linear scale, the upper part on a logarithmic scale. This option is useful for example for plotting a backgroundsubtracted spectrum. Often there are bins with a low count rate which scatter around zero after background subtraction; for those channels a linear plot is most suited as a logarithmic scaling cannot deal with negative numbers. On the other hand, other parts of the spectrum may have a high count rate, on various intensity scales, and in that case a logarithmic representation might be optimal. The mixed scaling allows the user to choose below which yvalue the plot will be linear, and also which fraction of the plotting surface the linear part will occupy. For an example see the figure below.