Create and register a visualizer

A Visualizer accepts some combination of QIIME 2 Artifacts and parameters as input, and produces exactly one Visualization as output. Visualizations are visual representations of analytical results (e.g., plots, statistical results, summary tables) and by definition cannot be used as input to other QIIME 2 methods or visualizers. Thus, visualizers can only produce terminal output in a QIIME 2 analysis.

Create a function to register as a Visualizer

Defining a function that can be registered as a Visualizer is very similar to defining one that can be registered as a Method with a few additional requirements.

First, the first parameter to this function must be output_dir. This parameter should be annotated with type str.

Next, at least one index.* file must be written to output_dir by the function. This index file will provide the starting point for your users to explore the Visualization object that is generated by the Visualizer. Index files with different extensions can be created by the function (e.g., index.html, index.tsv, index.png), but at least one must be created. You can write whatever files you want to output_dir, including tables, graphics, and textual descriptions of the results, but you should expect that your users will want to find those files through your index file(s). If your function does create many different files, an index.html containing links to those files is likely to be helpful.

Finally, the function cannot return anything, and its return type should be annotated as None.

q2_diversity.alpha_group_significance is an example of a function that can be registered as a Visualizer. In addition to its output_dir, it takes alpha diversity results in a pandas.Series and sample metadata in a qiime2.Metadata object and creates several different files (figures and tables) that are linked and/or presented in an index.html file. The signature of this function is:

def alpha_group_significance(output_dir: str,
                             alpha_diversity: pd.Series,
                             metadata: qiime2.Metadata) -> None:

Register the Visualizer

Registering Visualizers is the same as registering Methods, with two exceptions.

First, you call plugin.visualizers.register_function to register a Visualizer.

Next, you do not provide outputs or output_descriptions when making this call, as Visualizers, by definition, only return a single visualization. Since the visualization output path is a required parameter, you do not include this in an outputs list (it would be the same for every Visualizer that was ever registered, so it is added automatically).

Registering q2_diversity.alpha_group_significance as a Visualizer looks like the following (find the original source here):

plugin.visualizers.register_function(
    function=q2_diversity.alpha_group_significance,
    inputs={'alpha_diversity': SampleData[AlphaDiversity]},
    parameters={'metadata': Metadata},
    input_descriptions={
        'alpha_diversity': 'Vector of alpha diversity values by sample.'
    },
    parameter_descriptions={
        'metadata': 'The sample metadata.'
    },
    name='Alpha diversity comparisons',
    description=("Visually and statistically compare groups of alpha diversity"
                " values."),
    citations=[citations['kruskal1952use']]
)

See the text describing registering methods for a description of these values.