https://bugs.documentfoundation.org/show_bug.cgi?id=110993
stragu <[email protected]> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |[email protected] --- Comment #28 from stragu <[email protected]> --- The code that defines how splines are calculated mention that it follows the ODF specification 1.2: https://opengrok.libreoffice.org/xref/core/chart2/source/view/charttypes/Splines.cxx?r=e871c9cb#532 The relevant section of the ODF 1.2 standard is section 20.26 chart:interpolation : http://docs.oasis-open.org/office/v1.2/os/OpenDocument-v1.2-os-part1.html#__RefHeading__1419480_253892949 (first dot point is for b-splines, second dot point is for cubic splines) What needs to be checked is that the calculation is implemented in LibreOffice does indeed follow the standard (or maybe if it needs to be updated to a more recent ODF specification, if the method has changed since 1.2). As stated in my comment 25, there are different methods to compute a cubic spline. LibreOffice seems to be method called "normal" in R, in which there will be negative values extrapolated to fit the data you are using. It seems it is expected to have negative values extrapolated in some cases, to fit a smooth line to the data. For example: - https://www.wavemetrics.com/forum/general/spline-interpolation-introduces-negative-values - https://stackoverflow.com/questions/56903631/interpolating-using-a-cubic-function-gives-a-negative-value-for-probability#comment100352184_56903631 - natural cubic splines to fit a sequence of number between 0 to 1, negative values created depending on number of degrees of freedom: https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/s12874-019-0666-3/figures/4 - https://stackoverflow.com/a/51426181/1494531 I know Regina commented on that section of the specification. Would you be interested in this particular issue, Regina? -- You are receiving this mail because: You are the assignee for the bug.
