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docs: update function docs on deriv 

Signed-off-by: mtfoley <mtfoley.mae@gmail.com>
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Matthew 2021-12-22 21:30:14 -05:00 committed by mtfoley
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docs/querying/functions.md
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@ -130,6 +130,11 @@ delta(cpu_temp_celsius{host="zeus"}[2h])
`deriv(v range-vector)` calculates the per-second derivative of the time series in a range `deriv(v range-vector)` calculates the per-second derivative of the time series in a range
vector `v`, using [simple linear regression](https://en.wikipedia.org/wiki/Simple_linear_regression). vector `v`, using [simple linear regression](https://en.wikipedia.org/wiki/Simple_linear_regression).
The range vector must have at least two samples in order to perform the calculation. The linear
regression is used to calculate a slope and offset for a trend line, and the slope value is used
for the derivative calculation. In order to reduce calculation errors, the timestamp of the first
sample in the range vector is set as `x=0` within the linear regression algorithm. When `+Inf` or
`-Inf` are found in the range vector, the slope and offset value calculated will be `NaN`.
`deriv` should only be used with gauges. `deriv` should only be used with gauges.
@ -329,6 +334,13 @@ January etc.
`predict_linear(v range-vector, t scalar)` predicts the value of time series `predict_linear(v range-vector, t scalar)` predicts the value of time series
`t` seconds from now, based on the range vector `v`, using [simple linear `t` seconds from now, based on the range vector `v`, using [simple linear
regression](https://en.wikipedia.org/wiki/Simple_linear_regression). regression](https://en.wikipedia.org/wiki/Simple_linear_regression).
The range vector must have at least two samples in order to perform the
calculation. The linear regression is used to calculate a slope and offset
for a trend line, and the slope and offset are used to calculate the predict
the value of the time series. In order to reduce calculation errors, the
timestamp of the first sample in the range vector is set as `x=0` within the
linear regression algorithm. When `+Inf` or `-Inf` are found in the range vector,
the slope and offset value calculated will be `NaN`.
`predict_linear` should only be used with gauges. `predict_linear` should only be used with gauges.