> OctoPerf's load testing - Line Chart Report Item documentation page. Line Chart Report Item edition and usage.

Source: https://api.octoperf.com/doc/analysis/edit-bench-report/report-items/line-chart/

# Line Chart Report Item

Line charts allow to draw statistics evolution over time, with a [band](#spread) showing how stable each curve is, or [against another metric](#x-axis) such as the number of virtual users.

![Line Chart Report Item](https://api.octoperf.com/doc/analysis/edit-bench-report/report-items/img/line-chart-item.png "Line Chart Report Item")

Several metrics are available, please report to the [Hit Metrics Availability](https://api.octoperf.com/doc/analysis/edit-bench-report/performance-metrics/#hit-metrics-availability) and [Monitoring Metrics Availability](https://api.octoperf.com/doc/analysis/edit-bench-report/performance-metrics/#monitoring-metrics-availability) tables. 

!!! note
    To customize a specific curve, please refer to the documentation page about the [legend and metrics](https://api.octoperf.com/doc/analysis/edit-bench-report/metrics-selection/).

## X axis

By default, a line chart shows its curves over time. The **X axis** box of the item **Configuration** panel lets you change that in two ways:

- **Keep time, and add a band** around each curve, to see how stable a metric is over the test: set a [Spread](#spread).
- **Plot the curves against another metric**, to answer questions such as _how does the response time grow with the number of virtual users?_

![X axis of a line chart plotted against a metric](https://api.octoperf.com/doc/analysis/edit-bench-report/report-items/img/line-chart-x-axis-form.png "X axis of a line chart plotted against a metric")

To plot against a metric, switch **Plot against** from _Time_ (the default) to _Metric_, then pick the **Type** and the **Counter** of the X axis:

- A **Load** metric, for instance the number of active users,
- Or a **Hits** metric, for instance the hit rate, the throughput or the errors per second.

Each curve is then drawn against this metric, one point per range of X values. The X axis has no filters of its own: for each curve, it is measured with the filters of that curve (virtual user, location, action...).

![Response time against the number of virtual users](https://api.octoperf.com/doc/analysis/edit-bench-report/report-items/img/line-chart-x-axis-load.png "Response time against the number of virtual users")

### Binning

When you plot against a metric, the X values are split into ranges. Each point of a curve is the average of that curve over the sampling intervals of the test, a few seconds each, whose X value falls into the range:

- **Automatic**: OctoPerf chooses the ranges from the values measured,
- **Fixed size**: the ranges have the width you set in **Bin size**, starting from 0. The bin size must be greater than 0.

The bin size is in the unit of the X metric, shown next to the field: users for the load, milliseconds for the response time, the latency and the connect time, bytes for the sizes. A chart may show times in seconds, but the bin size of a time metric stays in milliseconds:

| X metric                     | Ranges you want                 | Bin size              |
|------------------------------|---------------------------------|-----------------------|
| Number of users              | 0 to 10 users, 10 to 20 users... | `10`                  |
| Latency, around 1 second     | 10 milliseconds wide            | `10`, not `0.01`      |

Use a fixed size to get one point per load step, when your test increases the load by steps of a known size.

### Spread

An average can hide how much a curve moves around it. **Spread** draws a band around each curve, whether the chart is plotted against time or against a metric:

- **None**: no band, the curves show their usual values,
- **Standard deviation**: from one standard deviation below the average to one above it. The band never goes below 0,
- **Min / Max**: from the lowest to the highest value.

What the band covers depends on **Plot against**:

- **Against a metric**: the values of the range, that is the sampling intervals whose X value falls into it.
- **Against time**: the values of the [span](#span) around each point. Each point also becomes the average of its span: as long as a band is shown, each curve is a moving average. It is smoother than the raw curve, and a peak that lasts a single interval is flattened. To read the raw values, set **Spread** back to _None_.

![Response time over time with a standard deviation band](https://api.octoperf.com/doc/analysis/edit-bench-report/report-items/img/line-chart-x-axis-time.png "Response time over time with a standard deviation band")

!!! note
    A line chart measures the test in sampling intervals of a few seconds, and draws one value per interval. The band is computed from these interval values, not from the individual requests: a single slow request only raises the average of its interval, so it does not widen the band by itself. To study the distribution of the response times themselves, use a [Percentiles chart](https://api.octoperf.com/doc/analysis/edit-bench-report/report-items/percentiles-chart/).

### Span

On a chart plotted against time with a spread, **Span** sets how many sampling intervals each point takes on each side: at least 1, and 3 by default. Next to the field, OctoPerf shows the length of an interval when all the curves share the same one.

A span of 3 covers 7 intervals, the point's own interval and 3 on each side. With intervals of 5 seconds, each point and its band cover 35 seconds of the test. A larger span gives a smoother curve and a band over a longer period; a smaller span stays closer to the raw values.

Keep in mind that:

- At the start and the end of the test, or of the [time range](https://api.octoperf.com/doc/analysis/edit-bench-report/configuration/) shown, a point has fewer intervals on one side, so its span is shorter.
- An interval without measurements is not replaced by another one further away: the span simply holds fewer values.
- During a ramp-up, the values of a span differ partly because the load changes. The band then shows that trend too, not only how unstable the metric is.
- In a [comparison report](https://api.octoperf.com/doc/analysis/compare-bench-results/), each curve counts its span in the intervals of its own test. The number of intervals is the same for every curve, but their durations may differ.

### Rules

When you plot against a metric:

- The metric of the X axis cannot also be a curve of the chart: the curve editors no longer offer it once it is the X axis.
- A **Hits** X axis only accepts **Hits** curves. To plot HTTP, error, monitoring or custom metrics, use a **Load** X axis.

When a rule is broken, the **X axis** box explains which curve or metric to change.

### What changes on the chart

Against a metric:

- [Thresholds](https://api.octoperf.com/doc/monitoring/threshold-usage/#line-charts) are still drawn as horizontal lines. Alarms are not shown, since they are located in time.
- The [Y axes](#axes) settings, logarithmic scale and dedicated axis, still apply to the curves.
- While the test is running, the whole chart is computed again at each refresh, as new measurements can change every range.

Against time with a spread, the X axis stays a time axis: thresholds and alarms are shown as on any line chart.

!!! tip
    A typical use is a test that increases the load by steps: plot the average response time against the number of users, with a standard deviation band, to see at which load the response time starts to grow. The default report does it for you, with three charts plotted against the number of users:

    - **Response Time vs Load**, with a standard deviation band,
    - **Hit Rate vs Load**, the hit rate and the error rate,
    - **Connect Time / Latency / Response Time vs Load**, in the **SLA** section.

    A [comparison report](https://api.octoperf.com/doc/analysis/compare-bench-results/) gets a **Response Time vs Load** chart too, with one curve per test. You can add **Response Time vs Load** to any report from the Line Charts group of the [available items](https://api.octoperf.com/doc/analysis/edit-bench-report/#available-items), and some [insights](https://api.octoperf.com/doc/analysis/edit-bench-report/report-items/insights/#show-more) open these charts.

## Y axes { #axes }

When the curves of a line chart have very different magnitudes, you can keep them readable in a single chart from the **Y axes** box of the item **Configuration** panel:

- **Logarithmic scale**: an axis can switch from linear to logarithmic, for all the curves it carries.
- **Dedicated axis**: a curve can be detached to an axis of its own, which fits the values of that curve.

![Line chart with a dedicated axis](https://api.octoperf.com/doc/analysis/edit-bench-report/img/axes-dedicated-axis.png "Line chart with a dedicated axis")

For example, to show the minimum, average and maximum response times together, detach the minimum response time or switch the _Seconds_ axis to logarithmic.

This also applies to the line charts created from the monitoring. Refer to the [Y axes documentation](https://api.octoperf.com/doc/analysis/edit-bench-report/metrics-selection/#axes) for details.

## Zoom

You can draw a rectangle anywhere in the graph to zoom on that section:

![line-chart-zoom](https://api.octoperf.com/doc/analysis/edit-bench-report/report-items/img/line-chart-zoom.png)

And later use the **Reset zoom** button to go back to full display:

![line-chart-reset-zoom](https://api.octoperf.com/doc/analysis/edit-bench-report/report-items/img/line-chart-reset-zoom.png)

## Export

To export the chart please refer to the [dedicated documentation page](https://api.octoperf.com/doc/analysis/export-bench-report/export-report-items/#exporting-charts).