Metric graphs 101: Timeseries graphs

This is the first post in a series about visualizing monitoring data. This post focuses on timeseries graphs.

Observability is not just about having monitoring data—that data must be easily available and interpretable. Choosing the right visualization for your data is an important part of providing human-readable representations of the health and performance of your systems. There is no one-size-fits-all solution: you can see different things in the same metric with different graph types.

To help you effectively visualize your metrics, this first post explores four different types of timeseries graphs, which have time on the x-axis and metric values on the y-axis:

• Line graphs
• Stacked area graphs
• Bar graphs
• Heat maps

For each graph type, we’ll explain how it works, when to use it, and when to use something else.

Line graphs

Line graphs are the simplest way to translate metric data into visuals, but often they’re used by default when a different graph would be more appropriate. For instance, a graph of wildly fluctuating metrics from hundreds of hosts quickly becomes harder to disentangle than steel wool. It’s nearly impossible to draw any useful conclusions about your systems from a graph like that.

When to use line graphs

What	Why	Example
The same metric reported by different scopes	To spot outliers at a glance	CPU idle for each host in a cluster
Tracking single metrics from one source, or as an aggregate	To clearly communicate a key metric's evolution over time	Median latency across all web servers
Metrics for which unaggregated values from a particular slice of your infrastructure are especially valuable	To spot individual deviations into unacceptable ranges	Disk space utilization per database node
Related metrics sharing the same units	To spot correlations within a system	Latency for disk reads and disk writes on the same machine
Metrics that have a clear acceptable domain	To easily spot unacceptable degradations	Latency for processing web requests

When to use something else

What	Example	Instead use...
Highly variable metrics reported by a large number of sources	CPU from all hosts	Heat maps to make noisy data more interpretable
Metrics that are more actionable as aggregatesthan as separate data points	Web requests per second over dozens of web servers	Area graphs to aggregate across tagged groups
Metrics that are often equal to zero	Metrics tracking relatively rare S3 access errors	Bar graphs to avoid jumpy interpolations

Stacked area graphs

Area graphs are similar to line graphs, except the metric values are represented by two-dimensional bands rather than lines. Multiple timeseries can be summed together simply by stacking the bands, but too many bands makes the graph hard to interpret. If each band is only a pixel or two tall, the information conveyed is minimal.

When to use stacked area graphs

What	Why	Example
The same metric from different scopes, stacked	To check both the sum and the contribution of each of its parts at a glance	Load balancer requests per availability zone
Summing complementary metrics that share the same unit	To see how a finite resource is being utilized	CPU utilization metrics (user, system, idle, etc.)

When to use something else

What	Example	Instead use...
Unaggregated metrics from large numbers of hosts, making the slices too thin to be meaningful	Throughput metrics across hundreds of app servers	Line graph or solid-color area graph to track total, aggregate value Heat maps to track host-level data
Metrics that can't be added sensibly	System load across multiple servers	Line graphs, or heat maps for large numbers of hosts

Bar graphs

In a bar graph, each bar represents a metric rollup over a time interval. This feature makes bar graphs ideal for representing counts. Unlike gauge metrics, which represent an instantaneous value, count metrics only make sense when paired with a time interval (e.g., 13 server errors in the past five minutes).

Bar graphs require no interpolation to connect one interval to the next, making them especially useful for representing sparse metrics. Like area graphs, they naturally accommodate stacking and summing of metrics.

When to use bar graphs

What	Why	Example
Sparse metrics (e.g. metrics tracking rare events)	To convey metric values without jumpy or misleading interpolations	Blocked tasks in Cassandra's internal queues
Metrics that represent a count (rather than a gauge)	To convey both the total count and the corresponding time interval	Failed jobs, by data center (4-hour intervals)

When to use something else

What	Example	Instead use...
Metrics that can't be added sensibly	Average latency per load balancer	Line graphs to isolate timeseries from each host
Unaggregated metrics from large numbers of sources, making the slices too thin to be meaningful	Completed tasks across dozens of Cassandra nodes	Solid-color bars to track total, aggregate metric value Heat maps to track host-level values

Heat maps

Heat maps show the distribution of values for a metric evolving over time. Specifically, each column represents a distribution of values during a particular time slice. Each cell’s shading corresponds to the number of entities reporting that particular value during that particular time.

Heat maps are essentially distribution graphs, except that heat maps show change over time, and distribution graphs are a snapshot of a particular window of time. Distributions are covered in Part 2 of this series.

When to use heat maps

What	Why	Example
Single metric reported by a large number of groups	To convey general trends at a glance	Web latency per host
	To see transient variations across members of a group	Requests received per host

When to use something else

What	Example	Instead use...
Metrics coming from only a few individual sources	CPU utilization across a small number of RDS instances	Line graphs to isolate timeseries from each host
Metrics where aggregates matter more than individual values	Disk utilization per Cassandra column family	Area graphs to sum values across a set of tags

Conclusion

By understanding the ideal use cases and limitations of each kind of timeseries graph, you can present actionable information from your metrics more clearly, thereby providing observability into your systems.

In the next article in this series, we’ll explore other methods of graphing and monitoring metrics, including change graphs, ranked lists, distributions, and other visualizations.