Multiplot fails if it is the first plot in the script

When a script has a multiplotas it's very first plot, the plot produced it's a regular plot, the second plot overwrites the first one withouth any error indication:

Let's use this small example:

import numpy as np
import gnuplotlib as gp
th = np.linspace(0, 2*np.pi, 100)
x1 = np.cos(th)
y1 = np.sin(th)
x2 = 2*np.cos(th)
y2 = 3*np.sin(th)

gp.plot(
    ((
        x1,
        y1,
        dict(
            legend = "Circle",
            _with = "linespoints dt 4 pt 4",
        )),
     dict(
         title="Circle",
         xlabel="X",
         ylabel="Y",
     )),
    ((
        x2,
        y2,
        dict(
            legend = "Ellipse",
            _with = "linespoints dt 4 pt 4",
        )),
     dict(title="Ellipse",
         xlabel="X",
         ylabel="Y",
     )),
    _wait=True,
    square=True,
    multiplot="layout 2,1"
)

This produces the following output:

As we see, the Ellipse plot has overwritten the Circle plot, totally ignoring the multiplot argument. No warnings, no errors. Just silently ignores the option.

However, if we add a regular plot before the multiplot:

import numpy as np
import gnuplotlib as gp
th = np.linspace(0, 2*np.pi, 100)
x1 = np.cos(th)
y1 = np.sin(th)
x2 = 2*np.cos(th)
y2 = 3*np.sin(th)
# Dummy plot so multiplot doesn't fail
gp.plot(
    np.sin(th),
    np.cos(th),
    _wait=True,
)

gp.plot(
    ((
        x1,
        y1,
        dict(
            legend = "Circle",
            _with = "linespoints dt 4 pt 4",
        )),
     dict(
         title="Circle",
         xlabel="X",
         ylabel="Y",
     )),
    ((
        x2,
        y2,
        dict(
            legend = "Ellipse",
            _with = "linespoints dt 4 pt 4",
        )),
     dict(title="Ellipse",
         xlabel="X",
         ylabel="Y",
     )),
    _wait=True,
    square=True,
    multiplot="layout 2,1"
)

We get the desired output

One curious effect is that this only works if _wait=True is provided in the Dummy Plot

首先运行 issue 中的两个 Python 复现，并跟踪项目对 plot 和 multiplot 的处理。比较第一个 plot 的情况与在其之前有一个 dummy plot 和 _wait=True 的情况。完成标准是：一个第一个 plot 使用 multiplot="layout 2,1" 的脚本能够渲染两个 plot，而不需要 dummy plot，也不会静默覆盖第一个 plot。