title: vis.gam transparent at top of "persp" graph
body: <p>I am running a GAMM using package mgcv. The model is running fine and gives an output that makes sense, but when I use vis.gam(plot.type="persp") my graph appears like this:
  <a href="https://i.stack.imgur.com/W1FHC.jpg" rel="nofollow noreferrer">enter image description here</a></p>
  
  <p>Why is this happening? When I use vis.gam(plot.type="contour") there is no area which is transparent.</p>
  
  <p>It appears to not simply be a problem with the heat color pallete; the same thing happens when I change the color scheme of the "persp" plot:
  <a href="https://i.stack.imgur.com/ZigA9.jpg" rel="nofollow noreferrer">persp plot, "topo" colour</a></p>
  
  <p>The contour plot is completely filled while the persp plot is still transparent at the top.
  Data:</p>
  
  <pre><code>      logcpue assnage distkm fsamplingyr
  1  -1.5218399       7  3.490        2015
  2  -1.6863990       4  3.490        2012
  3  -1.4534337       6  3.490        2014
  4  -1.5207723       5  3.490        2013
  5  -2.4061258       2  3.490        2010
  6  -2.5427262       3  3.490        2011
  7  -1.6177367       3  3.313        1998
  8  -4.4067192      10  3.313        2005
  9  -4.3438054      11  3.313        2006
  10 -2.8834031       7  3.313        2002
  11 -2.3182512       2  3.313        1997
  12 -4.1108738       1  3.235        2010
  13 -2.0149030       3  3.235        2012
  14 -1.4900912       6  3.235        2015
  15 -3.7954892       2  3.235        2011
  16 -1.6499840       4  3.235        2013
  17 -1.9924302       5  3.235        2014
  18 -1.2122716       4  3.189        1998
  19 -0.6675703       3  3.189        1997
  20 -4.7957905       7  3.106        1998
  21 -3.8763958       6  3.106        1997
  22 -1.2205021       4  3.073        2010
  23 -1.9262374       7  3.073        2013
  24 -3.3463891       9  3.073        2015
  25 -1.7805862       2  3.073        2008
  26 -3.2451931       8  3.073        2014
  27 -1.4441139       5  3.073        2011
  28 -1.4395389       6  3.073        2012
  29 -1.6357552       4  2.876        2014
  30 -1.3449091       5  2.876        2015
  31 -2.3782225       3  2.876        2013
  32 -4.4886364       1  2.876        2011
  33 -2.6026897       2  2.876        2012
  34 -3.5765503       1  2.147        2002
  35 -4.8040211       9  2.147        2010
  36 -1.3993664       5  2.147        2006
  37 -1.2712250       4  2.147        2005
  38 -1.8495790       7  2.147        2008
  39 -2.5073795       1  2.034        2012
  40 -2.0654553       4  2.034        2015
  41 -3.6309855       2  2.034        2013
  42 -2.2643639       3  2.034        2014
  43 -2.2643639       6  1.452        2006
  44 -3.3900241       8  1.452        2008
  45 -4.9628446       2  1.452        2002
  46 -2.0088240       5  1.452        2005
  47 -3.9186675       1  1.323        2013
  48 -4.3438054       2  1.323        2014
  49 -3.5695327       3  1.323        2015
  50 -1.6986690       7  1.200        2005
  51 -3.2451931       8  1.200        2006
  52 -0.9024016       4  1.200        2002
  
  library(mgcv)
  f1 &lt;- formula(logcpue ~ s(assnage)+distkm)
  m1 &lt;- gamm(f1,random = list(fsamplingyr =~ 1),
                   method = "REML", 
                   data =ycsnew)
  vis.gam(m1$gam,color="topo",plot.type = "persp",theta=180)
  vis.gam(m1$gam,color="heat",plot.type = "persp",theta=180)
  vis.gam(m1$gam,view=c("assnage","distkm"),
      plot.type="contour",color="heat",las=1)
  vis.gam(m1$gam,view=c("assnage","distkm"),
      plot.type="contour",color="terrain",las=1,contour.col="black")
  </code></pre>