Pine plot(), fill() and plotshape() in MQL5: indicator buffers

//@version=6
indicator("Bollinger Bands: plot, fill and plotshape", overlay = true)

len = input.int(20, "Length", minval = 1)
mult = input.float(2.0, "Standard deviations", minval = 0.1, step = 0.1)

[basis, upper, lower] = ta.bb(close, len, mult)

// a color that changes bar by bar
plot(basis, "Basis", basis > basis[1] ? color.teal : color.red, 2)
p1 = plot(upper, "Upper", color.gray)
p2 = plot(lower, "Lower", color.gray)
fill(p1, p2, color.new(color.gray, 90), "Band fill")
plotshape(close > upper, "Close above upper band", shape.circle, location.abovebar, color.red, size = size.tiny)

//+------------------------------------------------------------------+
//| plot(), fill() and plotshape() as MQL5 indicator buffers         |
//| Bollinger Bands from the built-in iBands handle.                 |
//| Custom indicator: save in MQL5/Indicators and compile (F7).      |
//+------------------------------------------------------------------+
#property indicator_chart_window
#property indicator_buffers 7
#property indicator_plots   5

// Plot 1 - fill(p1, p2): DRAW_FILLING uses two buffers. Drawn first so the lines sit on top
#property indicator_label1  "Band fill"
#property indicator_type1   DRAW_FILLING
#property indicator_color1  clrWhiteSmoke,clrWhiteSmoke

// Plot 2 - plot(basis, color = cond ? teal : red): DRAW_COLOR_LINE = value buffer + color index buffer
#property indicator_label2  "Basis"
#property indicator_type2   DRAW_COLOR_LINE
#property indicator_color2  clrTeal,clrRed
#property indicator_width2  2

// Plots 3 and 4 - plot(upper), plot(lower)
#property indicator_label3  "Upper"
#property indicator_type3   DRAW_LINE
#property indicator_color3  clrGray
#property indicator_label4  "Lower"
#property indicator_type4   DRAW_LINE
#property indicator_color4  clrGray

// Plot 5 - plotshape(close > upper, shape.circle, location.abovebar)
#property indicator_label5  "Close above upper band"
#property indicator_type5   DRAW_ARROW
#property indicator_color5  clrRed

input int    InpLen  = 20;  // Length
input double InpMult = 2.0; // Standard deviations

double FillUpBuf[], FillLoBuf[], BasisBuf[], BasisColorBuf[], UpperBuf[], LowerBuf[], SignalBuf[];
int    bandsHandle = INVALID_HANDLE;

int OnInit()
  {
   if(InpLen < 2 || InpMult <= 0.0)
      return INIT_PARAMETERS_INCORRECT;
   // Buffer order follows the plots: 2 for the filling, 2 for the color line, then 1 each
   SetIndexBuffer(0, FillUpBuf, INDICATOR_DATA);
   SetIndexBuffer(1, FillLoBuf, INDICATOR_DATA);
   SetIndexBuffer(2, BasisBuf, INDICATOR_DATA);
   SetIndexBuffer(3, BasisColorBuf, INDICATOR_COLOR_INDEX);
   SetIndexBuffer(4, UpperBuf, INDICATOR_DATA);
   SetIndexBuffer(5, LowerBuf, INDICATOR_DATA);
   SetIndexBuffer(6, SignalBuf, INDICATOR_DATA);

   PlotIndexSetInteger(4, PLOT_ARROW, 159);        // Wingdings dot, like shape.circle
   PlotIndexSetInteger(4, PLOT_ARROW_SHIFT, -10);  // above the bar
   for(int p = 0; p < 5; p++)
     {
      PlotIndexSetDouble(p, PLOT_EMPTY_VALUE, EMPTY_VALUE);
      PlotIndexSetInteger(p, PLOT_DRAW_BEGIN, InpLen);
     }

   // ta.bb(close, len, mult): middle = SMA, bands = SMA +/- mult x population standard deviation
   bandsHandle = iBands(_Symbol, _Period, InpLen, 0, InpMult, PRICE_CLOSE);
   if(bandsHandle == INVALID_HANDLE)
     {
      Print("Could not create the iBands handle, error ", GetLastError());
      return INIT_FAILED;
     }
   return INIT_SUCCEEDED;
  }

void OnDeinit(const int reason)
  {
   IndicatorRelease(bandsHandle);
  }

int OnCalculate(const int rates_total,
                const int prev_calculated,
                const datetime &time[],
                const double &open[],
                const double &high[],
                const double &low[],
                const double &close[],
                const long &tick_volume[],
                const long &volume[],
                const int &spread[])
  {
   if(BarsCalculated(bandsHandle) < rates_total)
      return prev_calculated;

   int start = prev_calculated > 0 ? prev_calculated - 1 : 0;
   int count = rates_total - start;

   // iBands buffers: 0 = BASE_LINE, 1 = UPPER_BAND, 2 = LOWER_BAND (oldest first in these arrays)
   double mid[], up[], lo[];
   if(CopyBuffer(bandsHandle, BASE_LINE, 0, count, mid) != count ||
      CopyBuffer(bandsHandle, UPPER_BAND, 0, count, up) != count ||
      CopyBuffer(bandsHandle, LOWER_BAND, 0, count, lo) != count)
      return prev_calculated;

   for(int k = 0; k < count; k++)
     {
      int i = start + k;
      BasisBuf[i]  = mid[k];
      UpperBuf[i]  = up[k];
      LowerBuf[i]  = lo[k];
      FillUpBuf[i] = up[k];
      FillLoBuf[i] = lo[k];
      // color = basis > basis[1] ? teal : red  ->  index 0 or 1 of indicator_color2
      BasisColorBuf[i] = (i > 0 && BasisBuf[i] > BasisBuf[i - 1]) ? 0.0 : 1.0;
      // plotshape(close > upper): a value where the shape goes, EMPTY_VALUE elsewhere
      SignalBuf[i] = close[i] > up[k] ? high[i] : EMPTY_VALUE;
     }
   return rates_total;
  }
//+------------------------------------------------------------------+