I see errors compiling shaders in the journalctl:
===
Jun 17 10:26:37 plasmashell[1283]: QOpenGLShader::compile(Fragment): 0(41) : error C1059: non constant expression in initialization
Jun 17 10:26:37 plasmashell[1283]: *** Problematic Fragment shader source code ***
                                   /*
                                    * This file is part of KQuickCharts
                                    * SPDX-FileCopyrightText: 2019 Arjen Hiemstra <ahiemstra@heimr.nl>
                                    *
                                    * SPDX-License-Identifier: LGPL-2.1-only OR LGPL-3.0-only OR LicenseRef-KDE-Accepted-LGPL
                                    */
                                   
                                   // This file contains common directives needed for the shaders to work.
                                   // It is included as the very first bit in the shader.
                                   // Important: If a specific GLSL version is needed, it should be set in this
                                   // file.
                                   
                                   // This file is intended for desktop OpenGL version 2.1 or greater.
                                   
                                   #version 120

Created attachment 129485 [details]
kquickcharts-5.71.0-pi-should-be-constant.patch

After applying this patch to the kquickcharts - issue is gone.

David, sorry for bothering you but there is another small issue.
I think this bug will affect only users with old enough video cards.

FWIW, I am seeing the same issue, black square instead of close button and pie chart timer. Please let me know if I can provide more info. I hope this helps.

$ sudo inxi -Fxx                     
[sudo] password for erinehart42: 
System:    Host: Marvin Kernel: 5.3.0-59-generic x86_64 bits: 64 gcc: 7.5.0 Console: tty 1 dm: sddm,sddm
           Distro: KDE neon User Edition 5.19                                                                                                                      
Machine:   Device: laptop System: LENOVO product: 43142MU v: ThinkPad T510 serial: R8322DB                                                                         
           Mobo: LENOVO model: 43142MU serial: 1ZHP102123Z BIOS: LENOVO v: 6MET91WW (1.51 ) date: 06/05/2012                                                       
           Chassis: type: 10 serial: R8322DB                                                                                                                       
Battery    BAT0: charge: 74.0 Wh 96.2% condition: 76.8/94.0 Wh (82%) volts: 12.4/10.8                                                                              
           model: Panasonic 42T4801 serial: 723 status: N/A                                                                                                        
           hidpp__0: charge: N/A condition: NA/NA Wh volts: NA
           model: Logitech Wireless Mouse serial: 4055-d2-2c-6b-b6 status: Discharging
CPU:       Dual core Intel Core i5 M 520 (-MT-MCP-) arch: Nehalem rev.2 cache: 3072 KB
           flags: (lm nx sse sse2 sse3 sse4_1 sse4_2 ssse3 vmx) bmips: 9575
           clock speeds: min/max: 1199/2400 MHz 1: 2710 MHz 2: 2780 MHz 3: 2649 MHz 4: 2638 MHz
Graphics:  Card: NVIDIA GT218M [NVS 3100M] bus-ID: 01:00.0 chip-ID: 10de:0a6c
           Display Server: X.Org 1.19.6 driver: nvidia Resolution: 1366x768@59.64hz
           OpenGL: renderer: NVS 3100M/PCIe/SSE2 version: 3.3.0 NVIDIA 340.108 Direct Render: Yes
Audio:     Card-1 Intel 5 Series/3400 Series High Definition Audio
           driver: snd_hda_intel bus-ID: 00:1b.0 chip-ID: 8086:3b56
           Card-2 NVIDIA High Definition Audio Controller
           driver: snd_hda_intel bus-ID: 01:00.1 chip-ID: 10de:0be3
           Sound: Advanced Linux Sound Architecture v: k5.3.0-59-generic
Network:   Card-1: Intel 82577LM Gigabit Network Connection
           driver: e1000e v: 3.2.6-k port: 1820 bus-ID: 00:19.0 chip-ID: 8086:10ea
           IF: enp0s25 state: down mac: 00:26:2d:f2:a3:9b
           Card-2: Intel Centrino Wireless-N 1000 [Condor Peak]
           driver: iwlwifi bus-ID: 03:00.0 chip-ID: 8086:0084
           IF: wlp3s0 state: up mac: 00:26:c7:20:46:96
Drives:    HDD Total Size: 1000.2GB (9.5% used)
           ID-1: /dev/sda model: TOSHIBA_HDWJ110 size: 1000.2GB serial: 373XC952T temp: 37C
Partition: ID-1: / size: 916G used: 89G (11%) fs: ext4 dev: /dev/sda1
RAID:      System: supported: N/A
           No RAID devices: /proc/mdstat, md_mod kernel module present
           Unused Devices: none
Sensors:   System Temperatures: cpu: 52.0C mobo: 0.0C gpu: 0.0:
           Fan Speeds (in rpm): cpu: 2731
Info:      Processes: 240 Uptime: 19:12 Memory: 4491.6/7821.9MB Init: systemd v: 237 runlevel: 5 Gcc sys: 7.5.0
           Client: Shell (sudo running in bash) inxi: 2.3.56

Thanks for the patch! Please submit it as a merge request at https://invent.kde.org/frameworks/kquickcharts/-/merge_requests/

Already did it :)

https://invent.kde.org/frameworks/kquickcharts/-/merge_requests/15

Thank you very much! Excellent first patch. May it be the first of many. :)

*** Bug 422680 has been marked as a duplicate of this bug. ***