Recent changes to 594: Windows 6.03: cannot create dotted plot

#594 Windows 6.03: cannot create dotted plot

2025-12-17T03:51:10.749000Z

Ticket moved from /p/gnuplot/bugs/2843/

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#2843 Windows 6.03: cannot create dotted plot

2025-12-14T08:26:05.420000Z

My personal preference is for rounded line caps; you can change that in the toolbar widget at the top of the plot window.

(Me too. But) I cannot see such an option. Only for “line ends”, not for the dashes.

Anyway, switching away “rounded ends” makes (1,1) very similar to ”what is expected from a densely dotted line”, for the cases of line widths 1, 1.8 and 2.5 I checked. I think this is a good starting point — and IMO one should try to make the case when “rounded ends” are enabled to have the look/density consistent with this case.

                        dashpattern.append( custom_dash_type->pattern[j]
                               * qt_optionDashLength * empirical_scale);

Well, at the least, for odd j one should subtract the line width if rounded line ends are enabled (or a suitable “empirical multiple” of the line width).

(BTW, it is not documented whether fractional lengths are enabled. — And with the current implementation it seems that they better should be.)

#2843 Windows 6.03: cannot create dotted plot

2025-12-14T07:02:43.057000Z

There are some terminal types where gnuplot has to construct the dots and dashes bitmaps empirically, but Qt is not one of them. Gnuplot passes through a linewidth and the requested array of lengths; Qt does the rest. What you see is what Qt provides. My personal preference is for rounded line caps; you can change that in the toolbar widget at the top of the plot window.

Qt documentation is here:
https://doc.qt.io/qt-6/qpen.html#pen-style

gnuplot code that calls it is here:

case DASHTYPE_CUSTOM:
       if (custom_dash_type) {
               QVector<qreal> dashpattern;
                for (int j = 0; j < 8 && custom_dash_type->pattern[j] > 0; j++) {
                        dashpattern.append( custom_dash_type->pattern[j]
                               * qt_optionDashLength * empirical_scale);
                }
               qt->out << GEDashPattern << dashpattern;
               qt->out << GEPenStyle << Qt::CustomDashLine;
        }
        break;

Windows 6.03: cannot create dotted plot

2025-12-14T04:07:23.331000Z

I fail to create usable dotted plots (with qtand other similar terminals). With lw 1 the results are practically invisible. With thicker lines (like lw 1.8), “the length of dots” seems to scale with line thickness — but the logic remains tuned up for hairlines, so “the length of the dot is too large”. (From what I saw how other people code it, the “length of the dot” relative its width should decrease to 1 (or maybe even to 0.8 for yet wider lines) when the width is “no more hairline”.¹⁾

¹⁾ Or, with round mitter, to 0.

Moreover, I’m not sure the dots are drawn with correct miters. With (1.1), I get a solid line²⁾, With (1.0), for many widths there are “skips”, but very narrow (sic! — worse than with (1,1)).

¹² Well, for some line widths, it does have hairline skips in it — problems with rounding?!

So there is a problem with calculation of length of “skips” as well…