ffmpeg: change alpha channel of a showwaves filter

I've been trying to figure this issue out for a few hours and I can't seem to find any solution.

I'm creating a video from an .mp3 and some images with the following command

fmpeg.exe -y -i temp\audio.mp3 -loop 1 -i Bokeh\frame-%03d.png -r 25 -filter_complex "[0:a] showwaves=size=1280x100:mode=line:r=25[wave];[1:v][wave] overlay=y=H-h:eval=init[canvas];[canvas]drawtext=fontfile='./tools/impact.ttf':fontsize=42:text='ORGANIKISMNESS':x=20:y=(h-170-text_h*2.20):fontcolor=white:shadowy=2:shadowx=2:shadowcolor=black,drawtext=fontfile='./tools/impact.ttf':fontsize=42:text='RETURN TO THE SOURCE PT.2 (ORGANIKISMNESS REMIX)':x=20:y=(h-170-text_h):fontcolor=white:shadowy=2:shadowx=2:shadowcolor=black" -shortest -acodec copy -vcodec libx264 -pix_fmt yuv420p -preset ultrafast -tune stillimage -crf 19 -movflags faststart "videos\Organikismness-Return to the Source Pt.2 (Organikismness Remix).mp4"

I'm trying to make the [wave] (showwaves) filter have some sort of alpha channel, to be slightly transparent to be overlayed on the rest of the video later.

So far I've tried the blend filter, but this complains that the sources are not the same size (one is 1280x720, the showwaves source is 1280x100).

I tried the colorchannelmixer filter, but I couldn't figure out how this should work.

Anyone has any idea how to do it?

FFmpeg colorkey filter

//colorkey=0x000000:0.1:0.5
[0:a] showwaves=size=1280x100:mode=line:r=25, colorkey=0x000000:0.1:0.5[wave]

0x000000 is 0xRRGGBB color ?
0.1 is Similarity percentage
0.5 is Blending percentage