title: Add gridlines to a output plot in Mathemaitca
body: <p>I'm trying to add gridlines to a output density plot in Mathematica. The plot is generated by a long calculation in Mathematica and when I do the plot I forget to add the Mesh->True options. I don't want to do all the calculation and generate the plot again since it takes a long time. Is that possible to add the gridlines or mesh lines to plot ONLY using the output plot at hand?</p>
  
  <p>For example, If I have a plot p. Is it possible to add the mesh lines ONLY manipulate p?</p>
  
  <p>In a ordinary one dimensional plot, this will work</p>
  
  <pre><code>p1 = Plot[Sin[x], {x, -3, 3}];
  
  Insert[p1, GridLines -&gt; Automatic, -1]
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/EGjGZ.png" alt="enter image description here"></p>
  
  <p>But when I try the density plot, it seems the gridlines is always under the plot, and can be seen only at the image margin area.</p>
  
  <pre><code>p2 = DensityPlot[Sin[x + y^2], {x, -3, 3}, {y, -2, 2}, PlotRangePadding -&gt; 0.2];
  Insert[p2, GridLines -&gt; Automatic, -1]
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/Qma0k.png" alt="enter image description here"></p>
  
  <p><strong>Updata</strong></p>
  
  <p>The Mesh option on the output plot will not work because Mesh is not a options of a Graphics:</p>
  
  <pre><code>Show[p2,Mesh-&gt;True]
  </code></pre>
  
  <p>will give a message "an unrecoginzed option name(Mesh) was encountered while rendering a Graphics"
  Thanks.</p>