MathGL draws intermittent line

[Denis Mihailovich]

This function draws a graph of a function in the memory:

    GetShape()->draw_func =
        [&](const Rect& r)
        {
            graph.NewFrame();
            graph.SetFlagAdv(1, MGL_NO_SCALE_REL);
            graph.SetScaleText(false);
            graph.SetSize(r.width, r.height, false);
            graph.SubPlot(1, 1, 0, "#");
            graph.InPlot(0.05, 0.95, 0.05, 0.95);

            graph.SetRanges(x_left, x_right, y_bottom, y_top);
            graph.SetFontSize(level);
            std::string f = "{" + format.color.ToRGB() + "}";
            graph.Axis("xy", std::string(f + "-1").c_str(), "h-1");
            if (format.grid_width > 0)
                graph.Grid("xy", std::string("h" + std::to_string(format.grid_width) + f).c_str());
            graph.SetQuality(MGL_DRAW_NORM);

            for (const Plot& p : plots)
            {
                auto& x = p.x;
                auto& y = p.y;
                if (!x.empty() && !y.empty())
                {
                    mglData x_data(x.size());
                    mglData y_data(y.size());
                    x_data.Set(x);
                    y_data.Set(y);
                    f = "{" + p.format.color.ToRGB() + "}-" + std::to_string(p.format.width);
                    graph.Plot(x_data, y_data, f.c_str());
                }
            }

            const unsigned char* picture = graph.GetRGBA();
            std::vector<unsigned char> arr(picture, picture + 4 * (graph.GetWidth() * graph.GetHeight()));
            window->DrawImage(r.left + 1, r.top + 1, r.width, r.height, arr);
        };

I get such a graph:

As you see the line is not very beautiful. How can I resolve it? I tried to set quality to MGL_DRAW_LMEM. The result is much better, but the grid is not shown. Is this issue about antialiasing? How can I turn it on or off? If width of graph >= 2, the line is solid, but I need to draw it in width = 1 also.