# Interpolation

## Fill

FunctionDescriptionExample
`fill()`Interpolate missing metric values for the metric.`<METRIC_NAME>{*}.fill(<METHOD>, <LIMIT>)`

The `fill()` function has two parameters:

• `METHOD`: The function to use as an interpolation method; choose from:

• linear: Gives you a linear interpolation between the beginning and the end of the gap.
• last: Fills the gap with the last value of the gap.
• zero: Fills the gap with a zero value.
• null: Deactivates the interpolation.
• `LIMIT` [optional, default=300, maximum=600]: The interpolation limit (in seconds) that represents the maximum size of a gap you want to interpolate.

Read Interpolation and the Fill Modifier for a detailed explanation of the `.fill()` function and its impact on interpolation.

## Default zero

FunctionDescriptionExample
`default_zero()`Adds a default value to sparse metrics.`default_zero(system.load.1{*})`

The `default_zero()` function fills empty time intervals using the value 0 or, if interpolation is enabled, with interpolation. Note: Interpolation is enabled by default for `GAUGE` type metrics. Like most functions, `default_zero()` is applied after time and space aggregation.

### Use cases

The `default_zero()` function is intended to address the following use cases (though it may also work for other use cases):

• Aligning gauges as 0 when performing arithmetic on sparse metrics (note: `COUNT` or `RATE` type metrics queried `as_count()` or `as_rate()` are always aligned as 0, so using `default_zero()` does not change how they are aligned; it only affects `GAUGE` type metrics).
• Resolving monitors from a no-data condition. This works for both simple and multi alerts, but the value 0 must not cause the monitor to trigger. For example, this would not work for a monitor with the query `avg(last_10m):avg:system.cpu.idle{*} < 10` because this monitor triggers (instead of resolving) when it evaluates to 0. Avoid using this function for error rate monitors with `as_count()` queries. See the as_count() in Monitor Evaluations guide for more details.
• Filling in empty intervals in sparse (but nonempty) series for visual reasons or to affect the min/max/average of a timeseries in a monitor evaluation. If the evaluation window doesn’t contain any points of data, `default_zero()` has no effect.
• Showing the value 0 on the timeseries widget when there is no data.

### Example

To demonstrate how the `default_zero()` function works, consider this single point created for a custom metric using DogStatsD:

``````\$ echo -n "custom_metric:1|g" | nc -4u -w0 127.0.0.1 8125
``````

When this metric is queried over the past 30 minutes, there is a single timestamp, because only one of the query’s rollup intervals has a point:

``````avg:custom_metric{*}

+---------------------+---------------+
| Timestamp           | custom_metric |
+---------------------+---------------+
| ---------           | ---------     |
| 2019-04-17 17:45:00 | 1             |
+---------------------+---------------+
``````

The `default_zero()` function interpolates this point five minutes forward in time (the default interpolation limit for gauges), then fills the remaining empty intervals with zeros:

``````default_zero(avg:custom_metric{*})

+---------------------+-----------------------------+
| Timestamp           | default_zero(custom_metric) |
+---------------------+-----------------------------+
| ---------           | ---------                   |
| 2019-04-17 17:30:00 | 0                           |
| 2019-04-17 17:31:00 | 0                           |
...
| 2019-04-17 17:44:00 | 0                           |
| 2019-04-17 17:45:00 | 1                           |
| 2019-04-17 17:46:00 | 1                           |
| 2019-04-17 17:47:00 | 1                           |
| 2019-04-17 17:48:00 | 1                           |
| 2019-04-17 17:49:00 | 1                           |
| 2019-04-17 17:50:00 | 1                           |
| 2019-04-17 17:51:00 | 0                           |
| 2019-04-17 17:52:00 | 0                           |
...
+---------------------+-----------------------------+
``````