Added an extended PCA example with the iris dataset and plotting (#56)

* Added an extended example with the iris dataset and plotting

docs/source/pca.rst

@@ -142,6 +142,46 @@ One can use the ``fit`` method to perform PCA over a given dataset.
     # reconstruct testing observations (approximately)
     Xr = reconstruct(M, Yte)
 
+**Example with iris dataset and plotting:**
+
+.. code-block:: julia
+
+    using MultivariateStats, RDatasets, Plots
+    plotly() # using plotly for 3D-interacive graphing
+
+    # load iris dataset 
+    iris = dataset("datasets", "iris")
+
+    # split half to training set
+    Xtr = convert(Array,DataArray(iris[1:2:end,1:4]))'
+    Xtr_labels = convert(Array,DataArray(iris[1:2:end,5]))
+
+    # split other half to testing set
+    Xte = convert(Array,DataArray(iris[2:2:end,1:4]))'
+    Xte_labels = convert(Array,DataArray(iris[2:2:end,5]))
+
+    # suppose Xtr and Xte are training and testing data matrix,
+    # with each observation in a column
+
+    # train a PCA model, allowing up to 3 dimensions
+    M = fit(PCA, Xtr; maxoutdim=3)
+
+    # apply PCA model to testing set
+    Yte = transform(M, Xte)
+
+    # reconstruct testing observations (approximately)
+    Xr = reconstruct(M, Yte)
+
+    # group results by testing set labels for color coding
+    setosa = Yte[:,Xte_labels.=="setosa"]
+    versicolor = Yte[:,Xte_labels.=="versicolor"]
+    virginica = Yte[:,Xte_labels.=="virginica"]
+
+    # visualize first 3 principal components in 3D interacive plot
+    p = scatter(setosa[1,:],setosa[2,:],setosa[3,:],marker=:circle,linewidth=0)
+    scatter!(versicolor[1,:],versicolor[2,:],versicolor[3,:],marker=:circle,linewidth=0)
+    scatter!(virginica[1,:],virginica[2,:],virginica[3,:],marker=:circle,linewidth=0)
+    plot!(p,xlabel="PC1",ylabel="PC2",zlabel="PC3")
 
 Core Algorithms
 ~~~~~~~~~~~~~~~~~