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Highcharts-style cubic spline interpolation

Source: R/approx.spline.R
approx.spline.Rd

Drop-in replacement for stats::approx. Uses the Catmull-Rom spline (with the same smoothing parameter as Highcharts type = "spline") instead of straight-line segments. The call signature and return value are identical to stats::approx, so it can be substituted transparently.

Usage

approx.spline(
  x,
  y = NULL,
  xout = NULL,
  n = 50,
  rule = 1,
  log = FALSE,
  smoothing = 1.5,
  ...
)

Arguments

x

Numeric vector of abscissa values.

y

Numeric vector of ordinate values, same length as x.

xout

Optional numeric vector of points where the spline is evaluated. If NULL, a regular grid of length n is used.

n

Integer. Number of points to generate when xout is NULL. Default is 50.

rule

Handling of points outside the range of x. 1 returns NA; 2 repeats the nearest endpoint value. Matches the semantics of stats::approx.

log

Logical. If TRUE, interpolation is performed in log-log space. Default is FALSE.

smoothing

Numeric tension parameter used in the Catmull-Rom to Bezier conversion. Default is 1.5 (Highcharts default).

...

Additional arguments ignored; present only for full signature compatibility with stats::approx.

Value

A list with components x and y, just like stats::approx.

See also

approx

Examples

x <- c(0.05, 0.10, 0.20, 0.30, 0.50, 1.0, 2.0, 3.0)
y <- exp(-x)
Td <- seq(0.05, 3.0, length.out = 100)
approx.spline(x, y, xout = Td, log = TRUE)$y
#>   [1] 0.95122942 0.92298087 0.90776491 0.89006474 0.85611788 0.81992556
#>   [7] 0.82664406 0.79806681 0.75706097 0.74457432 0.73689817 0.71800798
#>  [13] 0.69295627 0.66515366 0.63680013 0.60925220 0.61121472 0.60973509
#>  [19] 0.60268370 0.59140399 0.57703021 0.56049496 0.54254877 0.52378438
#>  [25] 0.50466179 0.48553158 0.46665573 0.44822536 0.43037563 0.41319798
#>  [31] 0.39675000 0.38106346 0.36879806 0.37502227 0.37866764 0.37994948
#>  [37] 0.37910191 0.37636695 0.37198593 0.36619303 0.35921049 0.35124547
#>  [43] 0.34248809 0.33311049 0.32326657 0.31309242 0.30270707 0.29221347
#>  [49] 0.28169979 0.27124064 0.26089839 0.25072453 0.24076082 0.23104054
#>  [55] 0.22158956 0.21242731 0.20356773 0.19502005 0.18678952 0.17887808
#>  [61] 0.17128488 0.16400679 0.15703885 0.15037462 0.14400647 0.13792591
#>  [67] 0.13868766 0.14387525 0.14800098 0.15105744 0.15306292 0.15405768
#>  [73] 0.15409982 0.15326124 0.15162364 0.14927488 0.14630573 0.14280707
#>  [79] 0.13886766 0.13457228 0.13000040 0.12522526 0.12031323 0.11532358
#>  [85] 0.11030843 0.10531289 0.10037541 0.09552814 0.09079749 0.08620458
#>  [91] 0.08176585 0.07749354 0.07339627 0.06947950 0.06574600 0.06219630
#>  [97] 0.05882902 0.05564130 0.05262900         NA