[examples] add example for manual error band plot

examples/fig21_error_band.nim

@@ -0,0 +1,43 @@
+import sequtils
+import plotly
+import chroma
+from std / algorithm import reversed
+
+# simple example showcasing error bands (by hand)
+
+let
+  d = Trace[float](mode: PlotMode.LinesMarkers, `type`: PlotType.Scatter)
+  size = @[16.float]
+d.marker = Marker[float](size: size)
+d.xs = @[1'f64, 2, 3, 4, 5]
+d.ys = @[1'f64, 2, 1, 9, 5]
+
+# Create a Trace for the error band
+let
+  dBand = Trace[float](mode: PlotMode.Lines, `type`: PlotType.Scatter,
+                       opacity: 0.75, # opacity 75% to be prettier
+                       fill: ToSelf, # `ToSelf` means the filling is done to its own data
+                       hideLine: true) # line width 0 disables the outline
+# Create X data that is first increasing and then decreasing
+dBand.xs = concat(d.xs, d.xs.reversed)
+# Assign the actual ribbon band. Currently needs to be a seq
+dBand.marker = Marker[float](color: @[color(0.6, 0.6, 0.6)])
+
+# define some errors we will use (symmetric)
+let yErr = d.ys.mapIt(0.25)
+# now create the first upper band range
+var yErrs = newSeqOfCap[float](d.ys.len * 2) # first upper, then lower
+for i in 0 ..< d.ys.len: # upper errors
+  yErrs.add(d.ys[i] + yErr[i])
+# and now the lower
+for i in countdown(d.ys.high, 0): # lower errors
+  yErrs.add(d.ys[i] - yErr[i])
+dBand.ys = yErrs
+
+let
+  layout = Layout(title: "testing", width: 1200, height: 400,
+                  xaxis: Axis(title: "my x-axis"),
+                  yaxis: Axis(title: "y-axis too"), autosize: false)
+  p = Plot[float](layout: layout, traces: @[d, dBand]) # assign both traces
+echo p.save()
+p.show()