title: Change font type and add sub-superscripts or greek letters in the labels of a corrplot correlogram
body: <p>I want to make a correlogram with the  <code>corrplot</code> R package.</p>
  
  <p>The data used are like the table below </p>
  
  <pre><code>head(dat)
      pH      EC     NO3    NH4    Alpha     Beta
  1 8.32 12.6174  5.5658 0.6406 641.1503 545.1455
  2 8.19 11.9373 10.5631 0.9045 754.5042 514.9512
  3 8.07 11.3534  6.8375 0.7977 744.9126 493.5152
  4 8.19 12.4137  9.9584 0.9023 879.1580 655.9412
  5 7.24  9.4181 10.0677 0.9382 637.0015 489.1772
  6 7.75 10.3971 10.6647 1.0393 756.1199 517.5440
  ....    ....
  ....    ....
  ....    ....
  </code></pre>
  
  <p>But i want the numbers in the  correlogram labels for NO3 and NH4 to be in subscript and for the Alpha and Beta to use the greek letters α and β.</p>
  
  <p>The code I used to make the correlogram is below</p>
  
  <pre><code>M &lt;- cor(dat) #create correlation matrix
  
  # colors to be used
  col2 &lt;- colorRampPalette(c("#67001F", "#B2182B", "#D6604D", "#F4A582", 
                             "#FDDBC7","#FFFFFF", "#D1E5F0", "#92C5DE", 
                             "#4393C3", "#2166AC", "#053061"))
  
  library(corrplot)
  
  corrplot(M,method="square", order="original", col=col2(20), 
              cl.length=11,type="lower",tl.pos = "lt",tl.cex = 1,
              tl.col = "black", tl.offset = 0.4, tl.srt = 90,cl.pos="r",
              diag = FALSE)
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/Z0F0I.jpg" alt="correlogram"></p>
  
  <p><strong>data</strong></p>
  
  <pre><code>dat &lt;- structure(list(pH = c(8.32, 8.19, 8.07, 8.19, 7.24, 7.75, 8.13, 
  8.06, 7.91, 6.77, 5.28, 7.89, 7.68, 6.23, 8.31, 8.38, 8.31, 7.12, 
  8.2, 8.33, 7.66, 6.24, 6.7, 7.84), EC = c(12.6174, 11.9373, 11.3534, 
  12.4137, 9.4181, 10.3971, 14.7648, 13.7368, 15.7797, 7.1344, 
  7.2042, 15, 11.7473, 8.087, 13.755, 13.5277, 14.8324, 8.0436, 
  14.4983, 13.6894, 16.1245, 7.1204, 7.4364, 13.4685), NO3 = c(5.5658, 
  10.5631, 6.8375, 9.9584, 10.0677, 10.6647, 11.6462, 12.5271, 
  10.9805, 7.4731, 9.9492, 10.7479, 11.6753, 10.4071, 11.3829, 
  12.2611, 11.9056, 10.2238, 9.8128, 11.1856, 4.3521, 4.6849, 7.3659, 
  9.8142), NH4 = c(0.6406, 0.9045, 0.7977, 0.9023, 0.9382, 1.0393, 
  1.1002, 1.1534, 1.2629, 1.0686, 1.0292, 1.1485, 1.058, 1.2891, 
  1.0845, 1.1024, 1.1585, 1.0358, 0.9188, 0.9402, 0.9176, 0.8957, 
  1.0508, 0.7891), Alpha = c(641.1503, 754.5042, 744.9126, 879.158, 
  637.0015, 756.1199, 888.1357, 928.22, 982.75, 683.9139, 729.0595, 
  764.2607, 673.8184, 493.8568, 708.114, 759.094, 1040.6623, 698.2495, 
  819.0605, 710.4485, 655.0136, 805.2089, 645.5548, 831.6886), 
      Beta = c(545.1455, 514.9512, 493.5152, 655.9412, 489.1772, 
      517.544, 588.3099, 579.361, 564.1002, 483.3981, 511.9086, 
      440.1535, 497.5929, 328.9879, 506.202, 505.5201, 636.8879, 
      494.4532, 461.9742, 439.8569, 492.6054, 549.7177, 428.9918, 
      540.8364)), .Names = c("pH", "EC", "NO3", "NH4", "Alpha", 
  "Beta"), class = "data.frame", row.names = c(NA, -24L))
  </code></pre>