More easily Plot Trades or Trade Signals

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**Is your feature request related to a problem? Please describe.**
Your addplot.ipynb page is phenomenal and I was able to figure out that I can add scatter points by;
1) Making a numpy array empty/zero
trs = np.zeros((length_to_add, 1))
2) Filling it with Nans
trs.fill(np.nan)
3) Setting my 2 scatter points as I know the index (x) and y values
trs[x] = y
4) Adding this to a dataframe
toplot.insert(6, "trades", trs, True)
5) Then using your well documented addplot:
apdict = mpf.make_addplot(toplot['trades'],scatter=True,secondary_y=False)
mpf.plot(toplot,type='candle',volume=True,addplot=apdict)

**Describe the solution you'd like**
While this is great it took a lot of energy - I'm moving from the plotpy (for the candles!) and there I simply wrote:
plt.scatter(trade_list_idx,trade_list_price,c='b')

Which was simpler than messing around with a dataframe and so on.

Start with addplot.ipynb and the documented make_addplot call shown in the issue, then review the existing plotting entry point for how addplot data is accepted. Done would provide a simpler way to plot trade or signal coordinates directly, without requiring the described NumPy array and dataframe preparation.

Scritto dal modello di indicizzazione a partire dal testo della issue.