Update readme to new chart interface

The HTML/JSON interface for creating charts was recently changed. This
updates the readme accordingly. Additionally, this adds missing
documentation about the allowed configuration options.

docs/README.md

@@ -5,14 +5,58 @@ This web application is built as a [Jekyll](https://jekyllrb.com/) site and is i
 ### Charts
 
 Each `.html` page in this directory contains one or more charts.
-Each chart is defined as an `<svg>` element with a few special attributes that define their behavior:
+Charts are dynamically created by [`charts.js`](assets/js/charts.js) from `<div class="chart-placeholder">` placeholders.
+
+#### Structure
+
+Charts generally have the following structure:
+
+```html
+<div class="chart-placeholder">
+	<h3>Title</h3>
+	<canvas
+		data-url="{{ site.dataURL }}/chart-data.tsv"
+		data-type="history"
+		data-config='{...}'
+		></canvas>
+	<div class="info-box">
+		<p>
+			Description of the chart.
+		</p>
+	</div>
+</div>
+```
 
-1. `data-url`: a `.tsv` file generated by the [updater](../updater/README.md) (URL to the data file to be rendered by the chart)
-1. `class`: the kind of chart that will be rendered (`chart`, `table`, or `collaboration`)
-1. `data-type`: the subtype of a chart (`history` or `list`, only applicable if `class` is set to `chart`)
+- `<h3>`: the title of the chart (required)
+- `<canvas>`: used to configure regular charts; change the tag to `<table>` for tables and `<svg>` for collaboration charts
+  - `data-url`: the URL to the `.tsv` file that shall be rendered by the chart (generated by the [updater](../updater/README.md))
+  - `data-type`: the chart type, supported types are:
+    - `history`: time series charts for tracking metrics over time
+    - `list`: horizontal bar charts for general numeric data
+    - `table`: tables directly rendered from the TSV source
+    - `collaboration`: chord diagrams used to visualize connections between entities
+  - `data-config`: chart-type-specific configuration options (see [option list below](#configuration-options))
+- `<div class="info-box">`: zero or more info boxes with descriptive texts
 
 For details on how each kind of chart is rendered, take a look at [`charts.js`](assets/js/charts.js).
 
+#### Configuration Options
+
+##### History Charts
+
+| option | values | description |
+|---|---|---|
+|  `series ` | array of strings | only include these data series and drop all others (referenced by TSV table headings) |
+|  `visibleSeries ` | array of strings | only show the listed data series and hide all others initially (referenced by TSV table headings) |
+|  `sliceData ` | array `[t0, t1]` | slice the data from the TSV file as if `data.slice(t0, t1)` was called |
+|  `aggregate ` | weekly | if set to `weekly`, aggregate the data by week by computing the sum of the values within each week |
+
+##### List Charts
+
+| option | values | description |
+|---|---|---|
+| `stacked ` | `true`, `false` | render the data series as stacked bars instead of showing multiple bars per row |
+
 ### Development
 
 To run the site locally, install [Jekyll](https://jekyllrb.com/).