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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"inconsistent behaviour of the errorevery option in pyplot.errorbar() to the markevery keyword","articleBody":"When I set `markevery` and `errorevery` keywords together in `pyplot.errorbar()`, it works only in the case for an integer parameter. However, though the `markevery` keyword in `pyplot.plot()` allows more options as an input ([None | int | length-2 tuple of int | slice | list/array of int | float | length-2 tuple of float]), `errorbar()` cannot handle such options.  It would be nice if `errorevery` can accept the same kind of inputs as `markevery`. \n\nHere is a piece of code which demonstrates the issue.\n\n``` python\nimport numpy as np\nimport matplotlib.pyplot as plt\n\n# make data arrays\nndata = 10\nx = np.linspace(0, 1, ndata)\ny = np.linspace(0, 1, ndata)\nyerror = np.ones(x.size)*0.1\n\n# plot\nfig = plt.figure(figsize=(12,12))\nax1 = fig.add_subplot(221)\nax1.errorbar(x, y, yerr=yerror, fmt='o')\nax1.set_title(\"plot all points\")\nax1.set_xlim(-0.1, 1.1)\nax1.set_ylim(-0.1, 1.1)\n\nindex_plot = [1, 2, 8, 9]\n\nax2 = fig.add_subplot(222)\nax2.errorbar(x, y, yerr=yerror, fmt='o', markevery=index_plot)\nax2.set_title(\"set markevery with an array of index\")\nax2.set_xlim(-0.1, 1.1)\nax2.set_ylim(-0.1, 1.1)\n\nax3 = fig.add_subplot(223)\nax3.errorbar(x, y, yerr=yerror, fmt='o', markevery=2, errorevery=2)\nax3.set_title(\"set markevery and errorevery with an integer\")\nax3.set_xlim(-0.1, 1.1)\nax3.set_ylim(-0.1, 1.1)\n\n# the following does not work\nax4 = fig.add_subplot(224)\nax4.errorbar(x, y, yerr=yerror, fmt='o', markevery=index_plot, errorevery=index_plot)\nax4.set_title(\"set markevery and errorevery together with an array\")\nax4.set_xlim(-0.1, 1.1)\nax4.set_ylim(-0.1, 1.1)\n```\n\nI got the following error message. \n\n``` python\n---------------------------------------------------------------------------\nValueError                                Traceback (most recent call last)\n\u003cipython-input-43-81e2a2ded427\u003e in \u003cmodule\u003e()\n     21 \n     22 ax4 = fig.add_subplot(224)\n---\u003e 23 ax4.errorbar(x, y, yerr=yerror, fmt='o', markevery=index_plot, errorevery=index_plot)\n     24 ax4.set_title(\"set markevery and errorevery together with an array\")\n     25 ax4.set_xlim(-0.1, 1.1)\n\n/usr/local/lib/python2.7/site-packages/matplotlib/axes/_axes.pyc in errorbar(self, x, y, yerr, xerr, fmt, ecolor, elinewidth, capsize, barsabove, lolims, uplims, xlolims, xuplims, errorevery, capthick, **kwargs)\n   2713             xuplims = np.asarray(xuplims, bool)\n   2714 \n-\u003e 2715         everymask = np.arange(len(x)) % errorevery == 0\n   2716 \n   2717         def xywhere(xs, ys, mask):\n\nValueError: operands could not be broadcast together with shapes (10,) (4,) \n```\n\nAs a kind of bi-product, I found that a bool array with a length of N is also accepted as a valid input for `markevery`, but it's not documented in the manual (http://matplotlib.org/api/lines_api.html#matplotlib.lines.Line2D.set_markevery).  For example, I can use `index_plot=[True, False, False, True, True, True, True, True, False, False]` in the example above. \n\nI'm using matplotlib 1.4.3 with Python 2.7.9.\n","author":{"url":"https://github.com/monodera","@type":"Person","name":"monodera"},"datePublished":"2015-05-17T17:05:47.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/4438/matplotlib/issues/4438"}