Merging upstream version 1.39.0.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+# Chart dimensions, contexts, and families
+
+While Netdata's charts require no configuration and are [easy to interact with](https://github.com/netdata/netdata/blob/master/docs/cloud/visualize/interact-new-charts.md),
+they have a lot of underlying complexity. To meaningfully organize charts out of the box based on what's happening in
+your nodes, Netdata uses the concepts of **dimensions**, **contexts**, and **families**. 
+
+Understanding how these work will help you more easily navigate the dashboard, 
+[write new alarms](https://github.com/netdata/netdata/blob/master/health/REFERENCE.md), or play around 
+with the [API](https://github.com/netdata/netdata/blob/master/web/api/README.md).
+
+## Dimension
+
+A **dimension** is a value that gets shown on a chart. The value can be raw data or calculated values, such as the
+average (the default), minimum, or maximum. These values can then be given any type of unit. For example, CPU
+utilization is represented as a percentage, disk I/O as `MiB/s`, and available RAM as an absolute value in `MiB` or
+`GiB`.
+
+Beneath every chart (or on the right-side if you configure the dashboard) is a legend of dimensions. When there are
+multiple dimensions, you'll see a different entry in the legend for each dimension.
+
+The **Apps CPU Time** chart (with the [context](#context) `apps.cpu`), which visualizes CPU utilization of
+different types of processes/services/applications on your node, always provides a vibrant example of a chart with
+multiple dimensions.
+
+![An example apps.cpu chart with many
+dimensions](https://user-images.githubusercontent.com/1153921/114207816-a5cb7400-9911-11eb-8800-06f60b745f9c.png)
+
+The chart shows 13 unique dimensions, such as `httpd` for the CPU utilization for web servers, `kernel` for anything
+related to the Linux kernel, and so on. In your dashboard, these specific dimensions will almost certainly be different.
+
+Dimensions can be [hidden](https://github.com/netdata/netdata/blob/master/docs/cloud/visualize/interact-new-charts.md#show-and-hide-dimensions) to help you focus your
+attention.
+
+## Context
+
+A **context** is a way of grouping charts by the types of metrics collected and dimensions displayed. It's kind of like
+a machine-readable naming and organization scheme.
+
+For example, the **Apps CPU Time** has the context `apps.cpu`. A little further down on the dashboard is a similar
+chart, **Apps Real Memory (w/o shared)** with the context `apps.mem`. The `apps` portion of the context is the **type**,
+whereas anything after the `.` is specified either by the chart's developer or by the [**family**](#family). 
+
+By default, a chart's type affects where it fits in the menu, while its family creates submenus.
+
+Netdata also relies on contexts for [alarm configuration](https://github.com/netdata/netdata/blob/master/health/REFERENCE.md) (the [`on`
+line](https://github.com/netdata/netdata/blob/master/health/REFERENCE.md#alarm-line-on)).
+
+## Family
+
+**Families** are a _single instance_ of a hardware or software resource that needs to be displayed separately from
+similar instances.
+
+For example, let's look at the **Disks** section, which contains a number of charts with contexts like `disk.io`,
+`disk.ops`, `disk.backlog`, and `disk.util`.  If your node has multiple disk drives at `sda` and `sdb`, Netdata creates
+a separate family for each.
+
+Netdata now merges the contexts and families to create charts that are grouped by family, following a
+`[context].[family]` naming scheme, so that you can see the `disk.io` and `disk.ops` charts for `sda` right next to each
+other.
+
+Given the four example contexts, and two families of `sda` and `sdb`, Netdata will create the following charts and their
+names:
+
+| Context        | `sda` family       | `sdb` family       |
+| :------------- | ------------------ | ------------------ |
+| `disk.io`      | `disk_io.sda`      | `disk_io.sdb`      |
+| `disk.ops`     | `disk_ops.sda`     | `disk_ops.sdb`     |
+| `disk.backlog` | `disk_backlog.sda` | `disk_backlog.sdb` |
+| `disk.util`    | `disk_util.sda`    | `disk_util.sdb`    |