Tikz，负轴缺少标题

Question

第一个有效，而第二个无效。我认为问题出在第二个轴中只有负值（其余完全相等）。有人能帮帮我吗？

以下是代码：

\begin{figure}
\begin{tikzpicture}[scale=0.7]
\begin{axis}[
   axis lines = center,
    xlabel = {$x$},
    ylabel = {$y$},
    height=10cm, width=10cm, grid=major,grid style={dashed, gray!30},
    xmin=-6, xmax=6, ymin=-1, ymax=100
    ]%,xtick={1,...,10},ytick={50,100,...,200}]
    \addplot[draw=blue,domain=-9:9]{3*2^x};
\end{axis}
\end{tikzpicture}
\caption{$f(x)=3 \cdot 2^x$} 
\end{figure}


\begin{figure}
\begin{tikzpicture}[scale=0.7]
\begin{axis}[
    axis lines = center,
    xlabel = {$x$},
    ylabel = {$y$},
    height=10cm, width=10cm, grid=major,grid style={dashed, gray!30},
    xmin=-6, xmax=6, ymin=**-100**, ymax=**1**
    ]%,xtick={1,...,10},ytick={50,100,...,200}]
    \addplot[draw=blue,domain=-9:9]{-3*(1/2)^x};
\end{axis}
\end{tikzpicture}
\caption{$f(x)=(-3) \cdot \big( \frac{1}{2} \big)^x$}
\end{figure}

Answer 1

问题在于这个数字非常高，正如周围的框架所显示的那样：

\begin{figure}
\fbox{
\begin{tikzpicture}[scale=0.7]
\begin{axis}[
    axis lines = center,
    xlabel = {$x$},
    ylabel = {$y$},
    height=10cm, width=10cm, grid=major,grid style={dashed, gray!30},
    xmin=-6, xmax=6, ymin=-100, ymax=0
    ]%,xtick={1,...,10},ytick={50,100,...,200}]
    \addplot[draw=blue,domain=-9:9]{-3*(1/2)^x};
\end{axis}
\end{tikzpicture}
}
\caption{$f(x)=(-3) \cdot \big( \frac{1}{2} \big)^x$}
\end{figure}

其结果为：

问题显然出在域 上-9:9。 值x=-9对 产生了一个非常大的负值y，这导致了错误的边界框。 这可能是一个错误，但可以通过使用-6:6域（与 重合xmin:xmax）轻松规避：

\begin{figure}
\fbox{
\begin{tikzpicture}[scale=0.7]
\begin{axis}[
    axis lines = center,
    xlabel = {$x$},
    ylabel = {$y$},
    height=10cm, width=10cm, grid=major,grid style={dashed, gray!30},
    xmin=-6, xmax=6, ymin=-100, ymax=0
    ]%,xtick={1,...,10},ytick={50,100,...,200}]
    \addplot[draw=blue,domain=-6:6]{-3*(1/2)^x};
\end{axis}
\end{tikzpicture}
}
\caption{$f(x)=(-3) \cdot \big( \frac{1}{2} \big)^x$}
\end{figure}