René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Angled/Skewed Axis","articleBody":"Hello,\r\n\r\nI'd like to inquire about and offer to help add a feature I'd like to be able to use but I'm not quite sure where to start with implementing it. There's a scientific chart I'd like to make called a Skew-T Log-P diagram. The Y axis (Pressure as a vertical coordinate) is in logarithmic space, which thankfully core-plot already does! The strange part is that the X-axis is skewed (Temperature) anywhere from 33-45 degrees to the right of the vertical axis. I noticed that there is a not-yet implemented ScaleType identifier called 'angular', which sounds like it would be a good direction for being able to implement such a transformation. Here's an example plot below. The solid red line highlighted as the freezing level is an example of how a constant X axis value is represented by a slanted line. \r\n\r\n![image](https://user-images.githubusercontent.com/1411265/135560291-eb8685db-a70d-495d-9be0-72400c0bb5b7.png)\r\n\r\nThe Charts library for iOS has the ability to apply a matrix transform in which the underlying data can be skewed, but there's nothing for the axis lines. Awkwardly, Charts also doesn't support logarithmic data spaces easily. I would also prefer to use core-plot since it leverages the GPU more than Charts. I'd also prefer to be able to keep things in data-space coordinates and implement this at a lower level. \r\n\r\nI've implemented this a little more directly before using Qt Widgets using the following functions to transform from data space to pixel space. I understand that implementing this would be required at a lower level than what's done in Qt here, but if I can be directed to a good starting place I think I could figure out the rest and make a pull request. \r\n\r\nAny suggestions on where to start?\r\n\r\n\r\n```\r\ndef initUI(self):\r\n        ## These are some padding variables for axis drawing, and data-space limits on the graph\r\n        self.lpad = 30; self.rpad = 65\r\n        self.tpad = 20; self.bpad = 20\r\n        self.tlx = self.rpad; self.tly = self.tpad\r\n        self.wid = self.size().width() - self.rpad\r\n        self.hgt = self.size().height() - self.bpad\r\n        self.brx = self.wid ; self.bry = self.hgt\r\n        self.pmax = 1050.; self.pmin = 100.\r\n        self.barbx = self.brx + self.rpad / 2\r\n        self.log_pmax = np.log(self.pmax); self.log_pmin = np.log(self.pmin)\r\n        self.bltmpc = -50; self.brtmpc = 50; self.dt = 10\r\n        self.xskew = 100 / 3.\r\n        self.xrange = self.brtmpc - self.bltmpc\r\n        self.yrange = np.tan(np.deg2rad(self.xskew)) * self.xrange\r\n        self.clip = QRect(QPoint(self.lpad, self.tly), QPoint(self.brx + self.rpad, self.bry))\r\n        self.originx = 0. # self.size().width() / 2\r\n        self.originy = 0. # self.size().height() / 2\r\n        self.scale = 1.\r\n```\r\n\r\n```\r\n    def pres_to_pix(self, p):\r\n        scl1 = self.log_pmax - self.log_pmin\r\n        scl2 = self.log_pmax - np.log(p)\r\n        return self.bry - (scl2 / scl1) * (self.bry - self.tpad)\r\n\r\n    def tmpc_to_pix(self, t, p):\r\n         scl1 = self.brtmpc - (((self.bry - self.pres_to_pix(p)) /\r\n                              (self.bry - self.tpad)) * self.yrange)\r\n        return self.brx - (((scl1 - t) / self.xrange) * (self.brx - self.lpad))\r\n```\r\n\r\n\r\n\r\n\r\n","author":{"url":"https://github.com/keltonhalbert","@type":"Person","name":"keltonhalbert"},"datePublished":"2021-10-01T03:48:12.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":11},"url":"https://github.com/454/core-plot/issues/454"}