4 Reference |

Syntax |

Description |

Arguments |

pp= pgfplot_cs(matrix[ ,type,derived] );pp= pgfplot_cs(x,y[ ,type,derived] );

The *pgfplot_cs()* function constructs a cubic spline for
given points and returns a piecewise polynomial (pp) structure.

The *matrix* argument contains point data, one point per
line. For each point we have *x* and *y* value (columns 1
and 2). The third column -- if present -- contains values for the
first derivative d*y*/d*x* at the points. If you don't
specify values for all points, use NA for the points where
d*y*/d*x* is not known.

Instead of specifying d*y*/d*x* in the third column, you
can specify d*y*/d*x* in the *derived* argument.
This column vector must have the same length as the number of rows
in the *matrix*.

The column vectors *x* and *y* can be used instead of
the *matrix* to specify coordinates. The vectors must have
equal lengths.

The *type* argument specifies the spline type.

Name | Details |
---|---|

not-a-knot not_a_knot |
The default spline type. The third derivative is contiguous in the second and penultimate point. |

variational natural |
Natural spline. The second derivative is 0 in the first and last point. |

periodic | The first and second derivative of the last interval in the last point are equal to the first interval in the first point. |

fast | The spline is constructed without solving a linear system of equations. Quadratic polynomials are calculated for each triplet of contiguous points. For the middle point the first derivative of the polynomial is used as the first derivative of the spline. The first and last polynomial is used for the first and last point. The time required for the calculation increases only linearly with the number of points. |

fast-periodic | Combination of fast and periodic. |

Dirk Krause

Schmalkalden, 2017-09-24