Both 1D (histogram) and 2D (biaxial) flow plots can be placed individually, or generated and arranged based on annotation data as a pivot table.
Creating a biaxial plot or histogram:¶
- Click on Flow Plot in the toolbar.
- Click in the illustration to place the plot.
- In the sidebar, under FCS File, select the file you want to plot by clicking on the file name and then click on a selection in the drop-down menu.
- Under Population, click on the population name to display.
- Tip: When you mouse over a population, you can click on the button to show the gate on its parent population. If you click the population name, you'll see only the events from that population instead.
- To change the plot axes, click on the axis name under Y Channel or X Channel, and choose from the list of channels.
- Optional: To change the plot type (dot, density dot, contour, or histogram), go to Plot Settings and choose a different Plot Type.
- Optional: Adjust the plot styling.
Example: Gating Strategy¶
One common use of individual flow plots is for displaying a gating hierarchy. The above image was created with three individual flow plots.
To create the example above, a first flow plot was placed as described above. The settings for the left biaxial plot were:
- General ⮞ FCS file: For the purposes of example, we chose a stimulated sample with exhausted T cells.
- Population: Singlet Lymphocytes
- Y and X Channel: CD3 and CD8
- Gates ⮞ Gate Labels: Name
When making a gating strategy, it's often easiest to copy and paste the previous flow plot. The histogram was made by copying the previous plot, and then changing:
- General ⮞ Population: CD8 T Cells
- X Channel: PD1
- Plot Settings ⮞ Plot Type: Histogram
- Histogram Fill Opacity: 8
- Gates ⮞ Gate Label Size: 14
The final plot was a copy-paste of the histogram, with the following changes:
- Plot Settings ⮞ Plot Type: Dot - Density
- General ⮞ Population: PD-1+
- Y and X Channel: FSC-A and CD28
- Axes and Legend: Tick labels were turned off
As a final touch, arrows were added from the toolbar to indicate the relationship of the plots. Many other styling options are possible for flow plots.
Some data visualization options are common to all graph types, but there are also some options that are unique to flow plots.
Plot Settings offers several color scales for biaxial plots.
Contour plots also offer a black and white option.
Cividis's blue-yellow gradient has perceptually linear brightness. Cividis data visualization is optimized for people with or without color deficiencies, and converts to grayscale without loss of information.
For more information:
- Nuñez J. Anderton C., Renslow R. Optimizing colormaps with consideration for color vision deficiency to enable accurate interpretation of scientific data at https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0199239.
Flow plots can be visually smoothed by changing the Plot Settings. If the smoothing is zero, no smoothing is applied; higher numbers increase smoothing. Above are examples of smoothing settings of 0 (left) and 1 (right) for the same data.
Both undersmoothing and oversmoothing can lead to misleading graphs of data. Undersmoothed plots, especially on small data sets, will emphasize individual peaks, obscuring the similarities of close samples. However, smoothing values that are too high will inappropriately merge peaks, reducing or even eliminating distinct populations. Oversmoothing is especially a concern when positive and negative aren't clearly separated, which can be unavoidable for some studies.
The contour plots above, taken from the same FCS file, show how smoothing settings can obscure the data. The image on the left has no smoothing. The undersmoothing results in "cold spots" that make the plot hard to interpret. The plot on the right is oversmoothed, making it hard to distinguish whether the events are from the same population or not.
For more information, see https://aakinshin.net/posts/kde-bw.
The Gates section shows or hides gates on a flow plot, and adjusts the gate label size and content. By default, the gate label will display the percent of the parent population within a gate. This option can be changed to the gate name or a variety of statistics, including median, standard deviation, or event count.
Flow plots (especially when displayed in pivot tables) can be helpful for reviewing gating. To adjust a gate, right click on the plot and select View in gating.
Subsampling a population can be helpful if there are a large number of events to display or when you are comparing populations or samples of different sizes (e.g. you are making a plot overlaying basophils on neutrophils).
- Click on your preferred subsampling mode. Percentage chooses a percentage of the original population. Absolute Count restricts the number of events to a specific number.
- Click on Before Gating or After Gating to determine whether subsampling occurs on every event in the file, before gating hierarchy is applied (before), or only the events in the population displayed in the plot (after).
- Type a number into Subsample Percentage/Subsample Count (depending on whether you chose Percentage or Absolute Count) for the percentage or number of events to show.
- Optional: type a number into Random Seed. Choosing a seed will keep the selection of events consistent.
Improper use of subsampling can create false impressions of data by distorting the apparent abundance of different populations.
Contour plot styling¶
Contour plots have additional options, all available under Plot Settings in the sidebar.
Percentile Start specifies the first percentile band; a value of 10 means that events below the 10th density percentile will be shown as individual dots (outliers) instead of within a contour. Events below this threshold this will appear as black dots.
This example shows plots with percentile start of 1, 10, or 50. It also uses the unfilled Color Scale, which is unique to contour plots.
Percentile Step controls how large each contour is; a value of 10 will create contour bands that each contain 10% of the events in a plot.
Histogram opacity can be specified by entering the percent opacity into Plot Settings.